You need to change the type of the channels to ECOG or SEEG.
For an introduction to intracranial recordings in Brainstorm, I'd recommend you start by following the ECOG/SEEG tutorials, using the example datasets provided:
https://neuroimage.usc.edu/brainstorm/Tutorials/ECoG
https://neuroimage.usc.edu/brainstorm/Tutorials/Epileptogenicity