Create New Gmail Account
You have the choice to register either a normal Gmail or a Google Apps account.
Click on either picture to get to Google's site and apply for a new account.
Register here for a regular and free Gmail account. Your Gmail address will be like yourname@gmail.com.
If you prefer to have your own domain, then apply for a Google Apps account here. The Gmail address will then be yourname@yourdomain.com, where yourdomain.com will be a name you pick.
After successful registration, return to Mailplane and enter your new email address and password into either the Authentication dialog or Accounts preference page.