Extension only shows Help

This is a problem regularly reported: While Show seems to work fine in your document after installation, trying to use it in other docs fails, with the Show add-on menu only displaying the Help entry.

Solution

Multilingual instructions on Google Support pages

With your document opened, 

• • • Click Add-ons and then Manage add-ons.

    • Choose the Show add-on from the list.

    • Click Manage Click the three dots at the top right of the Show! picture and then Use in this document to turn the add-on on or off.

      • You might have to refresh the page for this change to take effect (Ctrl-r or Cmd-r)

Would this fail, please make sure that the application is activated in your account Security Preferences.

Short explanation:

When you install a Google Drive application Add-on, this extension is installed for all your documents, but enabled by default only in current and future docs you'll create.

Long explanation:

The short explanation above is only true for Add-ons that use dangerous scopes right upon opening a document. Show uses such a scope, for one very dumb thing: it associates one variable of its own, stored in your doc, to remember what your Show preference is for this document: Highlight option on, or off.

If this variable doesn't exist, it will try to create it right after you open a doc, and request immediate permission to "modify" this doc in order to simply write this On/Off flag, in an invisible variable area that is not part of a doc, but associated to it.

Google's safety choice for this has been the one explained above : consent is by default granted for future docs, but not for existing ones.

All details are explained here.