After installing the Addin (2.0.040811.01) for Office 2010 I got it shown in Options->AddIns but disabled. Enabling doesn't work, too, so no Mathematics tab is visible.
After some research I found out that the installer is buggy:
- in my case the system drive is E: and not C:!
- installer sets the default values of registry keys for Word and OneNote resp:
instead of "E:\WINDOWS\system32\mscoree.dll"
After correcting with regedit I got the Add-in working!