Had exactly this a few months ago when we moved to wimdows7 and using 6.1 but no plex7. Why, well the process of The system guys building the installations for us developers were cutting corners, they were missing out the registering of active x, model api and build manager. In the end the end there were no short cuts and CA supplies installation exe for good reason as it is not as simple as copying a few dlls. i suggest you install plex 7 using the setup exe supplied.
Or you need to look at manually registering C:\Program Files\CA\Plex\6.1\bin\ob610CB.exe the OBComBldLib i think.
see plex help for model api which will also be missing
"Registering the Model API
The registration of the API interface is performed automatically when you install CA Plex. Therefore it should not normally be necessary to register the interface manually. If necessary it can be registered from the command line as follows:
Plex.exe /RegServer
This registers the interface and makes the necessary changes to the Windows registry."
Maybe something like this
ADMIN RIGHTS
C:\Windows\system32
C:\Windows\system32>cd C:\Program Files\CA\Plex\7.0\bin
C:\Program Files\CA\Plex\7.0\bin>ob7000CB.exe /Regsvr32
you are most likely missing a windows registry setting 7.0
HKEY_CLASSES_ROOT\TypeLib\{5C42D10F-4806-4EE2-B685-9C9BD9D3A81E}