First, create a proper site menu: http://www.forum.ucoz.com/forum/8-5638-1

Second, you can edit Site Menu block just fine. It's located here:
Control Panel -> Customize Design -> Global blocks -> The first container.

First block from the top is 'Site Menu' block, if you created site menu properly.
If you wont create site menu in correct way, there wont be any change to block.

that's help for me for when i get lost.