I think if the current RB2 menu had the Rock Band Store as the 2nd or 3rd selection and in big red letters, then more people would know about it. And instead of calling it that something like "Get More Music".
Another problem is that casual gamers might not know to connect their unit to the internet. I have a friend that has a PS3 and she loves playing GH3. She didn't know if her PS3 was connected to the internet. Internet gaming is still new to the casual gamer. They probably even have a wireless router for their laptop and didn't realize that they could've d/led more. I mean I know people that don't know what HDMI is let alone if they're getting real HD from their SD box and HDTV.