Microsoft has removed the dynamic help feature from the new version of visual studio. It is somewhat sad. I've been a big fan of dynamic help and will miss that feature.
I suspect that we'll see an extension that adds this functionality back, but until then...
There is this like really old mechanism that does nearly the same thing. Just highlight the member in the text editor and hit the F1 key. This will bring up MSDN help for the member you have selected.
It isn't as slick as dynamic help. Dynamic help was smart enough to scan for related topics and display them in a ranked list for you to pick from, where the F1 mechanism only goes to the most likely topic.
There is a pretty decent discussion on the topic over at LeeDumond's blog that also includes information from a PM at Microsoft about it.
