In order to use Jace, you must first have a Java 1.6 or higher runtime installed. The Oracle (formerly Sun) Java and OpenJDK should both be suitable. You only need the runtime edition (JRE). The development kit (JDK) is only necessary if you plan to do anything with the emulator's source code.
If you're using Mac OSX, you should look for Java under software updates in /Applications/Utilities/Java (note: I don't have a Mac and have no way of verifying this. If someone wants to shoot over a screenshot or some description of this I'll post it here. Thanks!)
After you have Java 1.6 or higher installed, next you'll need to download "jace.jar" from the sourceforge page: http://sourceforge.net/projects/java-ace/
Depending on if Java is integrated in your desktop or not, you might be able to either double-click on the jace.jar file to start it, or you might have a context menu option if you first right-click (or option-click if your mouse is insufficient at performing right-clicks). You should have an option labeled something like "Open with Java Runtime" or similar. If you're a Linux user, you will most likely have to set the execute bit (+x) before this will work.
If all else fails, drop to a command propmpt and type the following from the directory where jace.jar resides: "java -jar jace.jar"
Assuming that you got the Jace window up, you should now hear a beep and see "Apple //e" at the top of the window. You should also see some help text in the screen as well. The on-screen instructions summarize some basic keystrokes, but the most important of which is F4 which opens the configuration window. The configuration window is arranged in two panels. The left panel shows a summary of all configurable objects, and the right side lists all configuration settings available for the currently selected object. Note that settings are not effective until the Apply button is clicked. If you click on another configuration object on the left after changing settings, you should see the previous object indicated in italics to inform you there are settings that are not yet applied.
At the present, there are many different keys that map to the Reset key of an Apple //e. F12, Pause/Break and Page Down all will trigger a reset condition as long as the control key is pressed. Pressing these keys without pressing control will have no effect, just as a real Apple //e acts. You can warm-start (reboot) the emulator by pressing Ctrl + Left ALT + Reset (where reset is one of the aforementioned keys). If you press Ctrl-Reset during boot, you should get a basic prompt (which is a ] with a blinking square cursor). If you type the following and press Enter: PRINT "HELLO WORLD!" Then it should respond by printing out the same message on the next line and so on.
If you are unfamiliar with Applesoft Basic, you can get more information here: http://en.wikipedia.org/wiki/Applesoft_BASIC
From the basic prompt, you can boot a disk directly with PR#6 or boot a hard drive image with PR#7 -- assuming of course that you've already inserted an image in the configuration screen.
When you press F4, a window will pop up showing the structure of the emulator on the left. When you click on one of the items on the left, configuration options for the selected device will open on the right.
Where to find softwareYou can find disk images (as well as some hard drive HDV and/or 2MG) all over the web. If you download a compressed file (gz or zip extension) then use a decompression utility (such as winzip) to extract the actual disk image before trying to use it in JACE. Here are a couple of my favorite places: