This is a legacy capability that (if you are new to NX) you should NOT be using.
Siemens has stated that (at some point after NX9?) the GPM post (which uses MDF/MDFA) will likely break and it will NOT be fixed once it breaks.
Why would you want to use it for a new post?
(and if you have a legacy post you are trying to maintain, I would suggest you re-do it as a UGpost post).
Production: NX10.0.3.5 MP5 + patch/TC11.2
I'd rather be e-steemed than e-diseaseled
Why would you want a MDF file?
As Ken said, this is obsolete. GPM postprocessing went on critical maintenance in 2003, and became unsupported in 2010. Please read SFB-NX-5292 for the details.