Is it written as partmgr or PartMgr? The latter is the correct one (and remember to reboot your computer if you have needed to make this change).
If this still doesn't work, and it sometimes doesn't, then there are no real options left for you. You could try reinstalling FSX again (but you may not want to in case you have many add ons that will all need reinstalling or to run the risk of another problem developing). Win10 has been quite troublesome in this regard for many.
You may know about FSX-Steam Edition. This is a good option at a reasonable price to replace FSX boxed version. Otherwise, for the extra cost, Prepar3d (by Lockheed Martin) is the way to go, as it's a 64 bit Sim and continually being updated and developed.
Chris