Connecting your accounts before you logged in with Google Drive
Just change your email address to your Google account address in your Personal Info before you access MindMeister via Google Drive and your accounts will be automatically connected.
Connecting your accounts after you logged in with Google Drive
If you already accessed MindMeister via Google Drive and you were using another email address in your MindMeister account than your Google email address a new account was created. You didn't lose your mind maps or subscription. Your accounts just need to be merged.
Switch to your Personal Info to click on "Merge Accounts" at the bottom which will connect your accounts.
You will then be prompted to enter the details of your original MindMeister account and your accounts will be merged.