My first hit on Google for "AddOnName" (YMMV) gives me the document
http://msdn.microsoft.com/library/en-us/devref/HTML2/DAR_Properties_(A)_373.asp
with the closing paragraph
Note Beginning with Microsoft Visio 2002, the AddonName property
cannot execute a string containing arbitrary VBA code. Code that was
formerly passed to the AddonName property can be moved to a procedure
in a document's VBA project that is called from the AddonName property.
See the archives of this newsgroup for more discussion.
--
Reinier
Welcome. It took me quite a while to figure this out myself.