You have to buy a mlbtv for the year if you want to watch games on your ipad or iphone.
If you get the mlb app for the iphone I know you get the first month free.
Mlbtv is around 100$ for the year I think.
There's also blackout restrictions for MLBTv so if you live in New York and want to watch yankee games on mlbtv it will be blacked out.