[Freesurfer] Manual intervention for skull stripping

Hi,

I'm a freesurfer newbie. I was following the instructions from the beginners guide to fix bad output from skull stripping. However, it didn't work as it should be as described below (see italics) when I clicked on my middle button. Is there any option that I could do to re-assign my buttons to perform the operation? Thank you very much for your help!

"Use Ctrl-1 and Ctrl-2 to cycle between the two volumes. Find a place in the image where the cerebellum is missing in the output volume, then use the middle button on the mouse to paint in cerebellum from the auxiliary volume. Cycle back and forth between the volumes frequently so you know where you are." -- extracted from the internet source.