Copy the mgz file somewhere else, rename it to remove the "aseg" word, so it is not interpreted as an ASEG atlas by Brainstorm.
Then Import surface > File format = Volume mask or atlas (subject space).
Downsample the resulting atlas before displaying it (it contains > 1 million vertices)
In the Scout tab, the atlas Structures contains the meshes.
It might be more meaningful to compute volume source models and import your atlas as ROIs