And I also did a fresh install of Minecraft 1.5.2 only to test the new Mo'Creatures. It was not to go wrong.
In single player, Mo'Creatures works correctly.
But I can not access the server.....
I hate it when something works fine in single-player but server related none-sense comes up. Ugh!
This may sound silly, but did you install the GuiAPI onto the server? Please say you didn't... XD
Also, is the server spitting an error as well when you try to login to it?
I'm afraid I won't be of much help as I've never had this particular issue myself when I was running a server with Creatures on it. If it's a brand new server, as of today. Try recreating the server? What I'm suggesting here is that you remove Creatures from the server completely. Why I'm saying this is, I am assuming perhaps you did two different downloads for MoC and put one of those downloads onto your server.
Try copying the MoC files you have on your client directly into the server itself? I'd imagine if the files are identical as to what you are running, or visa-versa you can't go wrong, right?