I was able to load the model in GMax exporting it to the autocad format but my NWMax plugin seems to be broken because I cannot export a .mdl even if is a .mdl directly imported from NWN without any changes, I cannot export it again.
Is the green NW icon in your system tray when gMax and NWMax are running?
If not, that would be the reason you cannot export. Not sure what version of NWMax you are using, but you could try looking in the gmax/scripts/nwmax directory for a file called nwmax.exe. Clicking on that should start snoop, the background program required to export from gmax. After starting snoop you should then see the NW icon in your system tray.
If you do see the icon in your system tray and it is green, then you have another issue and may need to re-install gmax and/or the NWMax scripts.
MDA