In order to use the plugin with your Irista user, you should authenticate Irista.
In the Publish Services section click on Set Up... that belongs to Irista.
The Lightroom Publishing Manager will show up, where you can sign in with your Irista user by clicking on Authorize.
An irista login application will pop up to help you to sign in with the Irista user where you would like to publish your photos.
Once the authentication is finished, the login application will disappear and you will see now in Lightroom, that you are authorized to use the plugin with your Irista user.
Click on Save, and you're ready to upload your photos onto Irista!