OK. I found the solution.
Indeed, the Designer HAS TO BE installed with the Documentum Desktop 5.3 SP4. Which installed the DFC 5. When installing Desktop 5.3 SP5 instead of SP4, it seems that the registry entries or files are missing and the Deisgner cannot find the Documentum Desktop installation (maybe something is different in the installation procedure of the Destkop application).
Then,...if you want installing DCTM Desktop 5.3 SP5 with Adobe Designer 7, I strongly recommend installing Desktop 5.3 SP4 first, then installing Adobe designer 7 with the DCTM plugin, next, uninstall SP4 and install SP5 in the same directory as the previous installation. I tried it and it worked perfectly!
Hope this helps some of you.
Regards,
Tanguy