MarketPress Plugin and Theme Selection

I am confused about the MarketPress plug in. I have set up my site using the WPMU Nelo Theme. Then when I go to the MarketPress plug in and start setting it up, it asks to select a theme, either SimpleMarket or FrameMarket/GridMarket. If I select this theme it overrides WPMU Nelo. Are these just theme suggestions? Or if I don't select one, it will use the current theme somehow? Or do the 2 themes somehow work together? (That doesn't appear to be the case) Thanks!