I've looked over other threads and can't find a definitive answer to adding the isthmian league. I've tried using the editor to add the league and once i've saved the changes they are not available on the game when I start it up. Any suggestions?
Here you go mate. In here you have all the leagues down to level 9 in English football. Just put it in sports interactive/football manager 2012/editor data. When you start the new game, tick the change box and after you start just select it. You'll have all the leagues loaded up and their clubs ready to get managed
Only thing I noticed with that database as i use it myself is no reserve squad (and as far as I know done nothing wrong and I haven't tampered with it):S
It unlocks everything down to level 10, plus some cups to keep the clubs away from bankrupting. So I'd say this is a better shout than the one I posted earlier. I tried it and it works fine. Sorry I didn't direct you to it earlier...