What you need to do is in the Browse Page, select the clip that you have already imported into the Media Pool and then select the matte that you want to associate it with in the storage browse area above, right click and select Add Matte.
The in the Color Page, add a node, right click and select the matte at the bottom of the menu. When you apply your color correction you will see the result. You can invert and blur the matte as well.
-chrispy