NX : To create datum plane form Part , Assembly in NX Siemens

To create a  datum plane from part in NX :

1. Right click the part and select Make work Part
2. Now, Go to Menu→Insert → Datum/Pint → Datum Plane
3. In the new window, select type as Inferred
4. Now, select the face in the part , where datum plane has to be created
5. If you are not able to select the face, ensure the face is a plane surface. If not, enable point on surface option in snap point tool bar
6. Now select any three points on the face to create the datum plane.
7. Press OK to finish the operation.
8. You can double click on the datum plane anytime to re-orient the position of the datum plane.

To create a datum plane in Assembly in NX

1. In Assembly navigator, select the assembly and make it as workpart
2. Now, Go to Menu→Insert → Datum/Pint → Datum Plane 
3. In the new window, select type as Inferred
4. Ensure that , Entire Assembly option is chosen , In top order tool bar
5. Now select the face of the part where datum has to be created