If you purchased a Pixlr Pro membership from Autodesk via pixlr.com, along the top of your browser window, click on Log in and enter your credentials.
Then, look for the Purchase History tab under the Account Settings drop-down option:
Note: If you purchased through the Mac App Store, you can view those details directly in iTunes. You can also cancel your subscription by visiting the "Account Info" tab. If you have any trouble with anything account related, feel free to shoot us a message here.