It usually happen if during the install an instance of VS is open.
in any case if you are working on VS2005 you need to make sure that the following files:
TypeMock.AddIn2005.AddIn
TypeMock.AddIn2005.dll
are in the VS add-in folder:
$(CommonAppDataFolder)/Microsoft/VisualStudio/8.0/AddIns
on vista for example this translate to:
C:ProgramDataMicrosoftVisualStudio8.0Addins
Also check the values in the registery under
HKEY_CLASSES_ROOTCLSID{E1A6A8C8-C7FA-4F97-843F-83F638EA4248}InprocServer32
and verify it points to TypeMock installation directory. (C:Program FilesTypeMockTypeMock.NET)
:!: if youre working on 64 bit machine registery and paths changes.