You can compile plugins within ImageJ using with either the Plugins->Compile and Run command or using the File->Compile and Run command in ImageJ's built in editor (Plugins->Edit). Both commands require that you download and install the
MRJ Software Development Kit (SDK) from

    http://developer.apple.com/java/text/download.html#sdk

The four QuickTime plugins are preinstalled in the Import and Save As
submenus. To compile or run these plugins you must first install 
QuickTime for Java by selecting "QuickTIme for Java" in the "Custom"
install option of the QuickTime 4 installer. QuickTime for Java is
preinstalled on Mac OS 9.04 and later.

The QuitHandler plugin in the Utilities folder is called by ImageJ to process the "About" and "Quit" commands in the Apple menu and to open files dropped on ImageJ and files with creator code "imgJ" that are double-clicked.