We worked on the import of the HCP datasets 5 years ago.
I think I remember there were many difficulties in reading the preprocessed files, as some data was missing and the structure were not standard.
We wrote a tutorial for downloading and importing the unprocessed data instead:
https://neuroimage.usc.edu/brainstorm/Tutorials/HCP-MEG
I tried to download the data again from the connectomeDB website, but it seems that something changed with the support of the Aspera plugin, I didn't manage to download the processed data for this subject.
Can you please share the processed .mat file that you are trying to import?
(upload it somewhere and post the download link here)
I'll see if I can do something simple to let you import this file in Brainstorm.
However, my general recommendation would be to work in Brainstorm starting from the unprocessed data.