"xmpp:tatoeba@chat.tatoeba.org?join" should ideally be displayed as a link which will open your XMPP client if you have one installed. Kind of like clicking on a "mailto: ..." link will open your email client. If you actually open a new tab and copy-paste the string in the URL bar, you should have a popup asking you which program to use.

But for this to really work, you need to make sure you have configured your client properly. Just like you cannot send emails before you configure your email account, you cannot join an XMPP chatroom before you configure your Jabber account.

To configure Gajim, from the menu: Gajim > Accounts > click on the "+" button.

(1) If you don't already have a Jabber account, Gajim actually has a list of servers where you can create an account. After you clicked the "+":
- Choose "I want to register for a new account" then click "Forward".
- Select a server. Not all of them work however, from what I remember.
- Enter a username and password.
- Then you'll have your account and will be automatically connected to it.

(2) If you already have an account, then:
- Choose "I already have an account".
- Enter your username, server and password.
- Click "Foward" then "Finish".

Once your account is setup in Gajim, you can use the "xmpp: ..." link and choose to open it with Gajim. Gajim will show you a popup simply asking for your nickname.

If you don't use the xmpp link, you would join the chatroom this way:
- from the menu: Accounts > Join group chat
- choose your nickname
- the room is "tatoeba", the server is chat.tatoeba.org, there's no password


Unless you will be very often in the chatroom, I would recommend to just use chat.tatoeba.org. From this web app, you don't need to worry about creating an account. Gajim is far from being user-friendly in my opinion.