Change Menu

I am using theRealDesign template for a site of mine. I am wanting to substitute the menu for SL Menu Pro. I have attempted disabling the template menu, and adding a Module in position "mainmenu" (as per the module positions here: http://demo.gavick.com/joomla25/thereal ... -positions) but that module position doesn't exist.
How do I get SL Menu Pro module into that module position?

mainmenu is not regular module position and you can't use it for any other content. The only way is to add new module position in mainmenu area and publish module there or use some regular template position. This positions are marked on module positions graphic with different colours to mark area where the built-it template code it displayed.