Mark Clip - X shortcut doesn't work for some reason. Is there an easy way to mark a clip without going into Source Monitor?

Hi guys,

I'm using Premiere Pro CC on Windows 8.1 Pro. The Mark Clip function - X shortcut key by default doesn't seem to work.

I know that if I go into Source Monitor I can mark the clip in there but WHY ? In the old day of CS5.5 I can mark a clip (not timeline) as quick indicator for frame / sound issue of a particular clip. One key press and I'm done.

So, is there an easy way to mark a clip in Premiere Pro CC without going into Source Monitor?

Cheers!