In the beginning of the movie, Sean (Josh Hutcherson) was chased by the cops, and got arrested. Why? It's because he accessed the government satellite. A young kid accessed a satellite? What for? That's because Sean received a signal from his grandfather, Alexander (Michael Caine) who was on his journey to find the mysterious island, so he needed the satellite to boost the power of his signal receiver :)
His mom's boyfriend, Hank (Dwayne Johnson), who at first didn't believe that the signal came from his grandfather, finally helped Sean in deciphering the code from his grandfather, and realized that The Mysterious Island existed. Sean then decided to help Hank to find his grandfather, hoping to fix the relationship with Sean.