You can export the models out of AC3D as .3DS and import then in gmax. Hope, that helps.
Edit:
After import in gmax, do a right click on the model and go to down to "convert To" and do a "convert to poly".
After that do a "convert a mesh" to convert back. It's because the 3DS-export of AC3D isn't that perfect and
you will not see the model in the orth-views without this little trick
BTW:
It's similar in TurboCad and so on: You cannot see the unrendered models without little tricks