I'm a trial account, Level 8, Rouge... It's says in my abilites book (Ambush: Go see your trainer). At this point I would have gone to him, Bought it, Select train and I was done... But there is nothing in the trainers stock, This is bugging me... Or it's a bug in WoW? Someone help? Or at least settle my nerves.

Check the drop list on the training screen where available skills are displayed, on the top right corner of the panel. Make sure "Available skills" is checked. If that doesn't work, open your spell book and check the "subtlety" tab and see if ambush is learned. Hope that helps.