Joining MUCs with AdiumX
What to enter
Simply type ⇧⌘J and enter these values:
Make sure that your Jabber account is selected in the drop down list at the top.
Without going cross-network
If you don't have a jabber account somewhere, you can use the Jabber access of psyced directly. Put psyced.org in the server field and allocate your nickname (one that isn't registered already..). Then you can join PSYC rooms in the same way, only the server is called psyced.org, or wherever else your PSYC room is located according to its uniform.