Before you can add a Mastodon profile, you first need to create and add a Mastodon application. If you want to add Mastodon profiles that are hosted on different Mastodon instances, you will need a separate Mastodon application for each instance.
To add your Mastodon profile, use Social Networks, Social Profiles, Connect Social Profile, Connect Mastodon Profile in the menu. Fill out the form and click the Authorize Access button. Then follow any prompts by Mastodon to log in and approve the required access.