I'm not in front of my home machine right now, but the option to cut an mkv with MKVToolnix is with the mkvmerge tool.
open mkvmerge, click "add" to find the file you want to cut (or drop the MKV onto the open window)
then I think you want the second/middle tab (can't remember what it's called, but it doesn't immediately make you think about editing files when you see the tab name)
Once your there, there is an option in about the middle of the screen for cutting using timecodes (and other methods).
I use the timecodes method to put any places i want a cut to be made, and you'll get a separate file for each side of a cut (2 cuts = 3 files)
I would put a timecode of 00:01:00 to gut it after 1 minute. If you watch the tool, or the folder it's copying/cutting to, you can cancel it after it moves to the next file, as you only want the first 1 minute file it's done with.
sorry I don't have names and details, but I hope that's enough to get you moving forward again.