Any errors in client or server console?
To elaboroate (you posted quicker than I)
It's difficult to help with so little information.
You're basically telling us "this command doesn't work".
We know it does for 99.999% of the rest.
Is this a listen server, or a dedicated server?
Is it Sandbox gamemode?
What version of ULX and ULib (latest release, or svn?)
Does your SERVER console have any errors at game startup?
Does your GAME console have any errors at server join?
Does your SERVER console have any errors at player join?
When you run the command, what does ULX tell you, if anything. (No access?)
Have you tried editing any code in /addons/ULX or /addons/ULib that might have caused this (and we mean ANY)?
Did you place your motd.txt you want displayed in /gmod/motd.txt ? (even if not, you should still see a ulx generic motd template)
Are you using -ANY- addons that replace motd?
What other addons are you running? (get the list from "ulx debuginfo")