If you're using keeperfx you can launch the game with the -alex command line option and press Ctrl+F10 on the map screen to proceed to the next level. In the original game I believe the only think you can do is use '-level 11' as a command line option, but I'm not sure that works.
lvl 10 is still quite an easy map though, a lot easier than lvl 11, so you'd be better off just telling us why you can't beat it and we can help you. Some tips though:
- You can take a creature with you from lvl9 which should help out.
- On the sides of the map (build a bridge) are increase level specials that give you a big advantage
- To win the map build a bridge to the blue keeper and kill him
- If you are attacked before you are ready, try to avoid the fight, claim the bridge(build a bridge towards if for example) and sell it so you can no longer be attacked.