I'm trying to color correct just a roadsign. So I send the clip from FCP to Motion and duplicate the layer. I select the top layer and press Option-R to select the Rectangle Mask Tool. The cursor changes into a cross. Now, when I click and drag to create a mask, upon release of the mouse button, the canvas turns black. I can see the handles of the mask, but that's it, nothing more. When applying a Levels filter the it doesn't have any effect at all.

Where am I going wrong?

I found a tutorial on the net on rippletraining.com ("Cops" revisited), and I do it just like that.

Hmmm. I just tried the same using a still and it worked like I expected it to work. The clip that it doesn't work with is encoded using Apple ProRes422 (original HDV 1080p25). Hints?

Just came off the phone with Apple support, and it might be a bug. This particular clip, when sent to Motion from the FCP timeline creates a project with this particular problem. Creating a new project in Motion and then importing the clip works fine.

I have to go through the whole she-bang with Apple now, to see if we can pin down the exact problem. Might be a couple of days but I'll post the results.