Here's an article that may assist you further: http://support.presonus.com/hc/en-us/articles/210050023-Studio-One-3-Invalid-Login-and-Email-Error-
. You account should have been created under the same login info that your Google Plus account is under, so you should be able to log in at my.presonus.com using that information.
If you're having trouble remembering your password, you can click on the "Forgot your password?" link to do a password reset. It will only let you reset the password for an e-mail address with an active account, so this is also a quick way to find out which e-mail is/is not registered.