• Download a messaging client compatible with XMPP (such as Pidgin, Gajim, Adium, Spark). You may want to look here) and install it.
  • Add the XMPP account that you want to use.
  • In the menus, try to find something like 'Join a Chat', 'Join Group Chat', or related.
  • Fill the fields Room and Server with the information above. In case there is only one field for both the room and the server, the format to use is 'room@server'.