Hey Paul,
any other thoughts on this query ?
I have the same issue. The cause in my situation is that the the addin is programmed for 32-bit Visio (I checked the PE header), and I have Visio 2010 64 bits on Windows 7, which won't work. Apparently, either the installer did not recognize this correctly, or there is no 64 bit version of this addin (which would be a pity, as I understand it, most Office installations nowadays are 64 bit).
Possibly, you have a 64 bit version too (even though you mention it is 32 bit, are you certain)?