You should know by now that Windows Phone "Mango" has Facebook Chat built in to the Messaging app (see our full Mango review). Turning it on is actually pretty simple: navigate to the Messaging (SMS) app, hit Setting and enable. Or you can do it under Settings --> Applications --> Messaging. Easy enough, so why the how to? Well, this is more a "troubleshooting" thing i.e. you're one of the very few who can't it to work.
See, since I've been running "Mango"  (nearly a month next week), I could not get Facebook Chat enabled. Yup. Every time I'd throw that lil' switch, it would ask me to setup Facebook Chat, it would then open what looks like an embedded IE window where it would return with a "Page cannot be found". Seriously. It did this dozens of times and I just thought "welp, it's not enabled yet".