All you have to do is make sure you are using cellular on your iPad/iPhone and download the MLB at bat app. Open it and you will be prompted to get a free sub for the current season.
I think it's the $2.99 per month account that is free. Once you create an account using your phone or iPad, you can use that to login on other devices too.
Nice, thanks for the heads up! Just loaded the app and signed right in without issue. I would love video but I seem to get a fair amount of games on WGN and I don't want to shell out $25 a month to get that on my iPad.