"Insert citation" not working (EndNote X2)

Admittedly, I am not an advanced user of this program, and need some advice. My ‘insert citation’ button on the toolbar is not available for use, and I can’t figure out why. I can use the insert citation function through EndNote Web in my word processor, but some of my references don’t show up there for some reason. Thought it might work to re-install, but no luck. I’m sure it is something obvious, but I’m not finding it!