See INSTALL.md.

Free: Go to the multiplayer screen and click the Altening button, paste the account token you have in Free account token field then click the Use free token button, if it shows a username it worked

Paid: Go to the multiplayer screen and click the altening button, go to The Altenings account section copy your API Token, paste it in the API Token field then click the Check API Token button. If it works you will see one of the buttons change to a generate button and you can then generate accounts(note the API Token does not save and you need to readd it everytime you start minecraft)