As usual, it will be helpful to know the full and exact versions of the software you're using - including the build number (go to Help - About... if needed).
I just upgraded to Trados 6.5.5. from Trados 5.0 on the weekend, including converting my old MultiTerm database into a new one.
What exactly do you mean by "a new one" - did you convert it to MTiX, or to MT 5.5?
When I open both the Trados Workbench and MultiTerm, however, the Workbench says that MultiTerm is not running (Error 50152).
Have you had a look at the relatedKnowledge Base article, which you will get by simply entering the error number?
In checking the Help listing for that topic it mentions something about a fuzzy index, but I am not seeing a "Create fuzzy index" option in the new MultiTerm menu like there used to be in the older version. All I see is "Reorganize Termbase" which I have tried several times, but this does not seem to solve the problem.
It would appear that you're using MTiX (again, which version?), in which case you need to set the Terminology Provider in the Workbench window (see the KB articles for detailed instructions).
On a general note, did you register the new software components after installation? (Start - Programs - Trados 6.5 - Register Trados 6.5)