This capability was first introduced in NX Nastran 11. Documentation is in the Release Guide:
The second paragraph on that page contains a link to a PDF file documenting the Pre and Post processing workflow in Simcenter:
Hint: You need to customise the ribbon to enable the NX Nastran Topology Optimization Group on the Optimization tab in the Pre/Post application.
If you've set up the solution as described in the previously referenced help articles (particularly the part about requesting ESE results and setting THRESH=0.0), and if the solution was succcessful, the Normalized Material Density results should be written to the .op2 file automatically.