A client sent me a dictionary file, .mtw. I suspect this is an older version of Trados Multiterm.
Correct - .mtw files were used up to MT 5.x.
My Trados Version is 6.5.5 (my previous version is 3), and I don't own 6.0. How do I get the dictionary to work with my version? Is there maybe someone who could convert it for me? And can it also be converted back for the client? Or is there any way to convert it at least into a text format?
You did not mention your MTiX version, but as you're using T6.5.5 (i.e. build 438/439 or 441), I would expect you to use MT 18.104.22.1687 or 22.214.171.1247. This includes a utility called MT Convert, which allows you to convert MT 5.x termbases into MTiX-compliant XML. The process creates a termbase definition (.xdt), which is used to create a new termbase matching the structure provided, plus an .xml file containing the entries.
You can export entries into MT5.x format, but do you really need to do that? (IOW: Do you have to deliver a termbase, too?)
TW 6.0 can read TM exports created under TW 6.5.5.
BTW, I read in another forum post something about necessary index files. I only have the .mtw file. Are there any files missing?
Only if you wanted to use the MT5.x termbase, IIRC.