First, you'll need to mark a character as your main. By default, Raider.IO marks whichever character on your account has the highest Mythic Plus Score as your main character. To manually select a different character, simply open your Account Settings, select the desired main character from your character list (labeled "Your Characters"), and check "Mark as Main Character". Don't forget to click "Save" at the bottom of the page to ensure that this selection sticks!
Once you have finished marking a character as your main, there are a few conditions that you must meet in order for your alt to show your main's score:
Your alt must be max level
Your alt must be connected to your Main. If done correctly, you will see text like Alt of listed next to your alt's Bio on their page. If you do not see this, then make sure your alt shows up as a Claimed Character and that your alt's Character Settings are set to allow it to Link to your Main Character.
Your alt must meet the current minimum score of 200, or you must be a Patron of Honored level or higher and have your Patreon account connected to your RaiderIO account (found Here).
Once all of those conditions are met, you will then need to wait for the next version of the addon to be updated. This next version will contain the information linking your alt to your main in-game.