When routing you want to add vias and the way to do this is using the '*' key on the numpad. That'll drop a via and cycle thru *signal* (routing and plane) layers. The +/- keys will cycle all layers. Live it, love it. Sans a numpad, hit the tilde key '~' and select the command Next Layer Num * from the menu. It'll achieve the same thing without a numpad. But you should really get a numpad. You can thank me in the comments.