I'm a long time MLB.tv subscriber. In the past Spring Training games have not worked on streaming devices like Roku, Apple TV, Tivo, and video game consoles until just before the first regular season game. They do work in the At Bat app and in a web browser.
I'm not sure why this is but they don't seem to update any of the home streaming device apps until right before the first regular season game. Keep this in mind when you guys are trying to get your Directv credentials working for these spring training games.