I'm trying to import a MS Access database (.mdb) in MultiTerm iX.
The steps are: open MultiTerm > Tools > Attach Termbase. Then I give the database a name and click OK.
Attaching means creating a link to a MTiX termbase - although MTiX uses .mdb files, their formatting is fundamentally different from any other database. IOW you can open an .mdb file using MS Access, but you cannot open any .mdb database in MTiX.
Maybe the original database has to be in a special form with special fields defined?
Exactly. MTiX comes with a converter utility called MT Convert, which you will find alongside MTiX. One of the supported formats is MS Excel, so perhaps you can export the tables in your Access database to Excel?