Using CS5 After Effects, rotobrush.
I used Rotobrush to isolate a section of my video and now I want to invert the rotobrush's mask.
In the compositin I see the mask. And I see Effects > Roto Brush > Matte, but I don't see a place to Invert the Mask.
How do I do that?
The Roto Brush does not create a mask. It creates a matte.
Here's a section from the Roto Brush section of After Effects Help that seems to do what you want:
In general, you can invert any matte by using it as a track matte and choosing Alpha Inverted Matte.
Please work your way through basic materials before tackling more advanced things like the Roto Brush effect.