01
Get on the tester list
Join the tester group. It is a public Google Group, and membership of it is what puts you on Play's tester list — so this is the whole of step one. If the group asks you to request membership rather than letting you straight in, it is approved by hand and you will get an email when it is done.
Join it with the same Google account you are signed in to on the tablet. Play matches the tester list against the account signed in to the Play Store, not against whoever opened the link — a different address is the single most common reason step 2 refuses somebody.