I am using Trados 6.5 Freelance and it supports only Multiterm 5 (.mtw).
Not at all - T6.5 FL does support MTiX, but you need to install it. (Have you installed MT5.5 at all?)
A client send a database in .mdb format. That is why I want to convert it.
Assuming that you have MT5.5 installed, you could ask your client to export the termbase in MT5.x import format.