That is a bit of an issue. I cannot see any keyboard keys bound to zoom in/zoom out. Often RTS games use Page Up/Down but this is not the case.
Currently key bindings are not user configurable. You could try a third party application to translate key presses into mouse wheel until this is implemented.
For example, I use EventGhost (http://www.eventghost.org/) on my media center, this could be configured to send mouse wheel commands when chosen keys are pressed.