Though I thought we had it in the readme, I see now we don't.
Try in your <gmod server>/ulx_motd.txt , not <gmod server>/addons/ulx.
If that doesn't work, try <gmod server>/motd.txt
I found a changelog mentioning we'll use ulx_motd.txt, but no inidication exactly where.
If neither of those work, on your server console, type "find motd", one of those will be a path statement showing where Gmod is showing from by default.