Thank you for the example file.
The old SON-2.32 library we were using to read the old .smr was not capable of reading the .smrx file, therefore I had to interface the new library MATCED available for download on the CED website:
http://ced.co.uk/upgrades/spike2matson
I pushed the modifications to the Brainstorm repository:
https://github.com/brainstorm-tools/brainstorm3/commit/b18a1bf642bf01b0c6745a30a8fc788bebb89375
Please update Brainstorm and try again.
Select the file format "EEG: CED Spike2 new 64bit" file format.
This is experimental, I only had your short example file to test the reader, so there might be some bugs left in the code. Be careful with the units, and the timing of the signals and the markers. Let me know if you observe anything unexpected.