The first thing you'll want to do is make sure you've tried the steps in this
sticky thread.
Just to double-check everything:
You have OmniFocus 1.7.5 installed, and the clip-o-tron has been installed from that version of the application?
The clipping shortcut you've specified in OmniFocus' preferences isn't already in use by Mail or some other application?
If those are both true, compare what you see in OmniFocus' preferences against what you see in the "services" section of the Keyboard pane of System Preferences. Does your chosen shortcut appear in both locations?