Hi JJ,
I don't know all the causes of this, but here are some I do know:
1. Invalid version of ReqPro (as you noted and not valid in your case).
2. The ReqPro project migrated to RRC has a different GUID than that of the ReqPro Project integrated with RQM. You can see this condition in a Firebug trace. If, for example, you took another baseline, the GUID is now different and when RQM does a check, it will fail.
3. RQM requirement links don't contain the ReqPro external requirement GUID. You could see this in the XML ("GET" with etlmode=true or abbreviate=false). I've only seen this with certain migrations like TM and it's not clear exactly how this happens. Re-synching can fix this. (That can be tough to implement for large projects.)
4. The project has 0 requirements, i.e. getReqProProjRequirementNum returns 0. Not sure under what condition this could do that, unless you indeed don't have any links. This check is for both collections/views and requirements.
5. Permissions - The user logged in must belong to JazzAdmins or JazzProjectAdmins and the user is an admin in the project area
Anything in the qm or rm logs that could give a clue?
Did you go through a migration previously that failed?
If you try adding one more new requirement (perhaps you can delete it after), does this trigger the button to appear?