Thanks so much for posting about this, and sorry for any confusion.
You can listen to Pandora for free from supported devices, like your new Apple laptop.
It looks like you wrote in about a complimentary session of Pandora Premium, which allows you to interact with an ad in order to listen to content on-demand.
To start a new session, simply search our library from your computer or mobile device for a song or album, and click play. If you are eligible for a free session of Pandora Premium, clicking play will trigger a short advertisement, after which you can enjoy your on-demand content.
Because these sessions are funded by advertisements, you can initiate them without worrying about being charged for Premium.
It looks like your account has already reverted to normal. To prevent this, you can listen to your normal stations versus clicking on individual songs to play.