The easiest way to install any addon is to use the Curse Client. It will automatically download and install everything you need. If you don't want to use it for whatever reason, you will need to download both a class module (e.g. Bitten's SpellFlash: DK), and SpellFlash.
It used to be required to also download Bitten's SpellFlash Library, but it is now included with each class module. If you downloaded and installed it in the past, it is now safe to delete.
If WoW says "Dependency Missing" next to your SpellFlash addons, something went wrong with installation. Some users even report this problem when using the Curse Client (which is supposed to take of all that for you). If this happens to you, SpellFlash did not get installed correctly. You will have to install it separately.
Yes, having multiple modules for a single class is perfectly fine. You can have both installed and choose between them in the options for SpellFlash by using the drop-down under "Selected Class Module". Get to the options by pressing escape (with default key bindings), selecting "Interface", going to the "AddOns" tab, then selecting "SpellFlash". Also notice many modules (including all of Bitten's) provide further customization by pressing "Configure Class Module."
If nothing at all ever flashes, it is probably either an installation problem or a Lua error. If you don't know what to do, follow these steps to narrow the problem down, then to either report it or fix it:
WoW has a difficult time with this particular error; it ends up blaming a seemingly random addon, instead of the one that actually caused the problem. The only way to determine whether SpellFlash (or one of its modules) is really at fault is to disable all the rest of your addons, then see if the error still occurs. You can turn addons on/off like this until you narrow down which one is actually causing the problem. I know that sucks to have to do, but I know of no other way.
This error is just like the one above.
When Blizzard releases a new content patch, they also usually classify any addons that have not been updated since then as "Out of Date." When this happens you will need to click the AddOns button on the character selection screen then click to "Load out of date addons." Often times this will make your SpellFlash modules work perfectly again. If you experience errors with SpellFlash in game after doing this, please open a ticket or leave a comment on that module's page so that it can be fixed quickly. If other "out of date" addons give you trouble after enabling them, you can return to that same screen and un-select those particular addons until they are updated (or uninstall them completely).
There are five possible reasons one of your spells may not be flashing, assuming your other spells are:
SpellFlash can flash your macro as long as you choose the question mark for its icon (which will automatically make the icon match whatever spell the macro will cast) and its first line is #showtooltip. Then it will flash as if it was the spell that shows up in the tooltip. Make sure "Flash Macros" is selected in the options. One user reports that macros managed by MacroBank are not flashing, but if he copies and pastes it into a new (duplicate) macro, that one works. If a macro is not working for you please try this workaround.
This sounds like a bug! It would be very helpful if you submitted a ticket including the Lua error. To see the error make sure they are turned on in your options (escape -> interface -> game -> help -> display lua errors), relog or /reloadui, then do whatever you did before to make it stop flashing. A window will pop up with a bunch of information you don't understand, but the author will understand perfectly. Please highlight all that confusing text, copy it (by pressing ctrl+c), create a ticket, and paste that information into it. Thank you!
Bitten's SpellFlash modules are designed for max level characters. They will all work before max level, but not at level 1. When exactly they will start working depends on your class, and I cannot say when that will be. In general, if you have most of the spells that are used for the rotation (you can find a list of them using the links at the top left of this website), it should be fine.
What is the difference between, e.g., BigRedBrent's SpellFlash: Warlock and Bitten's SpellFlash: Warlock?
SpellFlash does not do anything on its own; it requires a module to instruct it what to flash and when. Any addon author can create a SpellFlash module, even though one already exists for that class. To understand the differences you really need to check the descriptions of each one. As the author of Bitten's modules, I can say they are designed for dungeons and raids at max level, and are useful for dungeons as soon as you have the main spells that are listed for your spec in that module's description.
Bitten's modules replace the default Blizzard proc highlighting, rather than adding to them, because it is just too confusing to have Blizzard flashing some spells and SpellFlash flashing another. However, if you have "Default Proc Highlighting" enabled in the options for SpellFlash it will display a red border around spells to indicate procs. Also remember that Blizzard provides the option to show graphics in the middle of your screen to indicate procs (escape -> interface -> game -> combat -> show spell alerts). Be assured that the modules track your procs and take them into account when deciding which spell to flash.