Dear Angela,
you can also use MRIcroN to draw your own regions of interests or mask files or modify existing mask files on a voxel-by-voxel basis. However, if you want to mask the cerebellum (or any other region), then you should probably mask the normalised, unsmoothed data and then apply the smoothing kernel, so still during the preprocessing and not during model specification/estimation. Otherwise you are going to mask some of the relevant activation (which is smoothed towards/into the cerebellum to some extent, e.g. visual areas along the ventral surface).
Best,
Helmut
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