What operating system are you using and what version of Word?
I've never had it cause Word to crash. The 'insert citation' button is only available in Word once you've installed the relevant plug in (go to the 'Tools' menu on Mendeley desktop and I think the install option is there). Alas, this won't work if you're running Office 2010 64 bit version - they've been saying for a while that they're going to develop a plug in for that, but there's no news as to when.
I think Mendeley has a support forum so if people here can't help then try there.