How do you activate FaceTime if the menu item "FaceTime > Preferences" is dimmed out?

I am experiencing the issue described in http://support.apple.com/kb/TS3970, i.e. after upgrading my software FaceTime is no longer working and tells me that I should check my Internet connection although it is working perfectly. The strange thing is, that the menu "FaceTime > Preferences" is dimmed out, i.e. I cannot access it at all. Also, I have not clue as to how to find the hosts file described in the article mentioned above. Any suggestions/help? Thanks!