Integrating Pro Sites with mobile app offerings

I just signed up for the wpmudev program, and I have a fair amount of experience building single-site wordpress, but I'm missing something basic here. I tried simply installing pro sites with my WP 3.4.2 install and got the attached fatal error. I also noticed that I should be running some kind of multi-site option, but wordpress codex says it's been rolled into release 3 already. I feel like this is a really dumb question, but how do I activate multisites, and do I need to set it up from a clean install? the site I'm trying to add the pro sites feature is: http://mobileappxpress.com. we're selling mobile apps and mobilized webs, and we're finding that many customers need completely new standard webs as well, so it's natural that we offer that service. any advice appreciated (no audible snickering though). Thx, Chris Dollard