1) Can't open MW3 saves in Modio.You don't need one? That was just lupo's tutorial, there is a simpler much easier way to save it without a checksum.
1)Get your game save and open it in modio.
2)Go to File contents and Extract the save game from there onto your desktop ect.
3)THEN load it in MW2 cvar tool, add your mods.
4)And finally save it and replace the old file contents.
5)Rehash and resign, Done.
YOU NO NOT NEED A CHECKSUM TOOL.