There's another way. Just select the area you want to keep, and go to Edit>Crop Image or Edit>Crop selected bitmap (if you want to crop just the active layer).
Hope that helps!
I'm running into a similar issue. I'm using a new Mac and was using FW fine the other day. When I select crop it moves to the selection tool but when I click enter, nothing happens. No crop whatsoever.
I have the same problem. This works for me (Fireworks CS 5.1 and CS6 on Mac OSX 10.8.5): Instead of double clicking to crop, use the "enter/return" key!