I think most likely you're seeing that error message ('you've joined a class') in Minecraft because your 90-day access to our Minecraft server ran out. Everyone that sets up an account with our books should get unlimited access to our website and 2-3 months access to our minecraft server.
We've gone ahead and added 3 more months to your account.
If after 3 months you'd like to continue using our minecraft server, you can always buy another LearnToMod subscription. You can buy a 1-year subscription from learntomod.com -- we'll send an access key to the email you order with. Then, just login to your LearnToMod account as usual. In the middle of the LearnToMod homepage click the Account gear icon and then 'add time' and enter the new access key.