This example shows how to attach a context menu item for the "placeholder" plugin, as found in FCKEditor 2.3. This code will be very similar for your own plugins, just add to the your fckplugin.js file.
FCK.ContextMenu.RegisterListener( {
AddItems : function( menu, tag, tagName )
{
// under what circumstances do we display this option
if ( tagName == 'SPAN' && tag._fckplaceholder )
{
// when the option is displayed, show a separator the command
menu.AddSeparator() ;
// the command needs the registered command name, the title for the context menu, and the icon path
menu.AddItem( 'placeholder', FCKLang.PlaceholderDlgTitle, oPlaceholderItem.IconPath ) ;
}
}}
);