This is usually done by registring your action in the built in WSS.ACTIONS file.
WSS.ACTIONS file is a core file, that may be overriden in service packs or future fixes, and was not meant to be altered.
Instead, create a new file named *.actions (i.e. myActions.actions) in the same directyory as WSS.ACTIONS file, and add the following content to it:
<?xml version="1.0" encoding="utf-8"?>
And register your actions under
this way you avoide editing the SharePoint core files needlessly.