So you've verified that the dll is where it's supposed to be? Try registering it using regsvr32.exe to make Windows aware of it.
Sorry to know about your problem.
I would request you to check on more thing--
Whether you FrameMaker and RoboHelp belong to same installer i.e. were installed with TCS installation, as they would not show link option if installed individually with different installers.
If you are not sure, wipe out the complete TCS 2.5 and check if RH or Fm is installed, if yes, remove them and then reinstall the complete TCS 2.5 again on you machine. Launch Robohelp and test linking again.
If it still does not work, drop an email with some additional information - email@example.com