Which folder are plug-ins installed to?

Final Cut Pro X

FxPlug plug-ins are installed to:
~/Library/Plug-Ins/FxPlug

Motion templates are installed to:
~/Movies/Motion Templates/<PLUGINTYPE>/Digital Heaven/

Final Cut Pro (4-7)

When you select these options in the installer, the plug-in will be installed to the appropriate folder below:

Final Cut Pro (All Users)
Macintosh HD/Library/Application Support/Final Cut Pro System Support/Plugins

Final Cut Pro (Current User)
Home/Library/Preferences/Final Cut Pro User Data/Plugins

Final Cut Express (All Users)
Macintosh HD/Library/Application Support/Final Cut Express Support/Plugins

Final Cut Express (Current User)
Home/Library/Preferences/Final Cut Express User Data/Plugins