According to mlb.tv's website, it is only for iPhone, iPod touch, or select Android devices. If your device is not listed in there, I would imagine it is a compatibility issue. Is this on the Thunderbolt, or is this on the Imagio you have in your profile information? I would reach out to the website admins for help on this issue. I was unable to test this because it requires a subscription of either $49.99 or $89.99.
I hope that answers your question.