simple: Peek definition is a VS Pro+ feature I saw it in a version table on the Microsoft site, but I can't find a link anymore. (comparing express vs Pro vs ..).
Edit as of 16.11.2014: the new VS 2013 Community edition, which will replace the express edition longterm, does incorporate every feature the Pro edition does, so you get Peek definition for free (if you don't work on a closed source enterprise project in a company with more than 5 devs or more than 1 Mio. revenue/year).