MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
Hm... If the drive gear grinds at the filament, the motor seems to have enough torque. Otherwise the stepper would stall and loose steps. Did your last extruder have a MK8 drive gear, too? If you push on the idler, it moves some more and then stops. Does the motor stall in this case?
Could you post a picture or two of your extruder loaded with filament (e.g. as a make) for further inspection?