Hi!
Place a western unit on the map. Name him something you'll remember, like Frank. Save the mission to whatever name you want. Now, open your missions folder within your user folder. Should be here: C:\...\Operation Flashpoint\Users\YourName\Missions\
Open the folder that has the same name as the mission you just saved. In there you'll find a file called mission.sqm. Open it using Wordpad or Notepad. Next step is finding your guy, Frank, in the text. Simply search for Frank using the application's search function.
Frank will be displayed as follow: text="Frank";
If you look four lines above that line you'll see this: side="WEST";
That line is telling the game what side the unit Frank belongs to. Change the line to: side="EAST";
This makes him one of the Russians. If you would like him to be on the Resistance side you type GUER. CIV is for civilian.
Now, just save the mission.sqm in Wordpad or Notepad and load it up in OFP. Just one more thing. If you double-click on Frank after his side change he will become a Russian soldier model. So if you want to give him certain weapons or such, do this before you change his side.
Hope that helps. Cheers. ;D