Hi, doormouse:
If you have a file in .3dt format (not necessary with the exact extension), you can select FILE->IMPORT on IE3D/MGRID. Choose 3DT format and select OK. MGRID will prompt you for the file name with .3dt extension. You can enter *.* to list all file. Select the .3dt format file you want to import. MGRID will tell you how many lines are read. After you select ok, it will show you the imported geometry. I think it is straight forward.
best regards,