THE closest one is the Anaheim Station, right in the parking lot of Angel Stadium. You'll probably want to take a taxi, since there is a lot of construction in the area (they are tearing down some older hotels and the strip club, and redeveloping the area). It might be best to take a cab, it's about 2 miles, and should be a short cab ride. I've seen a lot of taxis there, so you should be OK..
As an alternate, you can exit at the Fullerton station, and take OCTA bus 43 right to the Disneyland entrance, 4.6 miles away. I hear it's even free. However, it's a little longer than taking the Anaheim Station.