An add-in that worked before suddenly disappears or ceases to work.
Office 2003 / Service Pack 3 (SP3) disables some add-ins
If you use Office 2003 / PowerPoint 2003 and have recently applied Service Pack 3 (SP3) or if you have automatic updates enabled in Windows and SP3 may have been applied for you, this Microsoft knowledgebase article explains what happened and gives several workarounds: