After entering Megaton and speaking with the Sheriff go to Moriarty's Saloon. After entering a man in a small room to your right will be waving you over. This is Mr. Burke, speak with him, agree to blow up the bomb. He will then give you Fusion Pulse Charge to install in the bomb. After installation go to Tenpenny Tower, if it is your first time here you will have to tell the guard Mr. Burke is expecting you. Then go in the gate, then the front door, straight back to the elevator. At the top make a left deal with another guard then enter Tenpenny's suit, go out to the balcony, watch a conversation, then detonate the bomb.