Quote: " i didnt get a serial code with it"
So you had the Free version, and you bought the build game upgrade from the TGC store? And it does not come with a serial code? I'm only asking because I did not go this route- I bought the full program. You may need to contact Support over this issue if you need a serial code and you don't have one from your purchase.
Quote: "when i try to save changes to the userdetails file it says access denied"
This is due to the fact that most people install FPSC in the default folder of C:\Program Files\The Game Creators\FPS Creator. The reason the access is denied is because Windows will not allow changes to files manually unless it (the OS) is doing them (particularly if UAC is turned on).
You can copy your user details file to another folder outside of the default (ex: c:\test) and edit the file. Once saved, you can right click on the file and select copy, then navigate to the proper location and click Control+V to paste. Usually you will receive a message it needs permission to copy, in which case you want to allow it to copy.

Twitter: @NFoxMedia