I'm here with a newbie question

I just got this game a week ago or so, and I like it a lot. The variety of historical scenarios and the possibility of playing almost any kind of historical battle out is really great.
However, while I enjoy playing scenarios, my main focus was on the scenario editor. I primarily bought this game to use the scenario editor.
I've made a few smaller scenarios (20 or so units) and I played them with the computer, it all seemed to work perfectly until I created a medium-sized scenario with a dozen or so towns, and around a hundred units. Upon entering the scenario selecting menu I realized that I couldn't play it with the computer.

I've searched the internet, and I understood that to play with the AI, I need to properly set up the Programmed Opponent.
I've given orders to each formation (Attack, Defend etc.) I made objectives and set up their values, I thought that it was ready to go. But apparently I don't know how to make a proper PO (and I didn't find much in the manual either) so I still can't play my scenario with the AI.
Can anyone explain how to create a proper, working PO in this game? Thanks
