[Strider]

You go through the battle listing as you do per single player. When you get to the one you want, you choose the button at the bottom that says "Start As PBEM".

You'll need to provide a unique file name for your battles. If you haven't done much PBEM turns, the most common format is

BattleName_Players_001

where the 001 number is incremented by each player when they save each turn they complete, so you guys can keep track of which file to load for which PBEM game.
You hit the spacebar, when you've finished your turn, and the window for file names will appear. I usually just click on the last file name for the PBEM game, then change the number, and then save it.

FYI:
Whoever starts the game, will play the side that is normally available when you start it as a single player, so if 'you' want to play the other side, you need to have the other player start the game.

Oh yeah...to load a PBEM game file, you have to scroll to the end of the battle listings to get the button "Load PBEM"...