This is a bit of a long shot but are there some FrameMaker Document settings in your Project Settings that are incompatiable with the FrameMaker version you are using. For example, is a FrameMaker template being used that is from a newer FrameMaker version.
Read the RoboColum(n).
I am having the same problem. I have a RoboHelp 8 project and want to import a file from Framemaker 6.0p405.
I even opened the Framemaker book and from there launched RoboHelp. I am still getting the message:
"This operation cannot be completed without FrameMaker. Please install FrameMaker and try again."
In the RoboHelp help, it says:
"FrameMaker book files created in a version earlier than FrameMaker 6 (BK) can only be imported into RoboHelp; they cannot be linked."
So, I'm assuming that I should be able to import a Framemaker 6 file into RoboHelp 8.
Have you tried saving the FM6 files as MIF and then importing them into RH8? That might work...