Download the latest Gajim release from their site and install it.
- Go to our registration page and register an account there
- Open the Gajim application
- Go in the Edit menu and Choose "Accounts"
- In the "Accounts" menu click on Add and select "I already have an account I want to use."
- On the next screen insert the Account details you specified during the registration remembering to insert lightwitch.org on the field after @
- You can either decide to "Remember your password" or not by checking or unchecking the appropriate checkbox
- Click next to commit your account creation
After you completed adding your account on Gajim you'll be able to log to the server and start communicating with your Jabber friends.
A few more references:
Joining a Multi-User Chat with Gajim: right-click on the "account name tab" select "Join groupchat" and enter the MUC Jid ("Jabber ID" the address for chats is usually in this format email@example.com).