Mine wont load on Steam

Certainly due to an incorrect registry registration of FSX Steam.

If you only have steam check
HKEY_LOCAL_MACHINE, "Software\Microsoft\Microsoft Games\Flight Simulator\10.0 SetupPath
or
HKEY_CURRENT_USER, "Software\Microsoft\Microsoft Games\Flight Simulator\10.0 AppPath

If you have Steam + a box edition of FSX then
HKEY_LOCAL_MACHINE, "Software\Microsoft\Microsoft Games\Flight Simulator - Steam Edition\10.0 SetupPath
or
HKEY_CURRENT_USER, "Software\Microsoft\Microsoft Games\Flight Simulator - Steam Edition\10.0 AppPath

If you don't have the above, you will have to repair your registry inscription. There's several tools for that

There's no SetupPath information in your HKEY_LOCAL_MACHINE, "Software\Microsoft\Microsoft Games\Flight Simulator\10.0 registry key
That is the reason
I'm quite surprised that the FS registry repair tools didn't correct that, especially the one referenced on this page
www.flight1.com/view.asp?page=library
Anyway, it is quite easy to add the information by yourself
Create a new String value (REG_SZ) and name it SetupPath
Then click Modify and enter as Value data the root path of your sim (by default: C:\Program Files (x86)\Steam\steamapps\common\FSX). If you installed it somewhere else use the custom path you defined