Hello,
If you run the segmentation of the MRI with FreeSurfer, you get access to a few atlases that can help you locate your region of interest: http://neuroimage.usc.edu/brainstorm/Tutorials/LabelFreeSurfer#Cortical_parcellations
But these are all anatomical atlases that are inappropriate to be used as is as regions of interest in your EEG or MEG analysis. You should be able to identify easily the section of the motor cortex involved in the movement from your recordings, as illustrated in the introduction tutorials (or the tutorials about median nerve stimulation). The SMA region, you can probably draw on the cortex from images you find online.
Alternative: In the fMRI literature, you can probably find MNI coordinates for the regions you are interested in, then create scouts starting from these coordinates using the menu Scout>New: coordinates from the Scout tab.
Francois