Instructions
Travel to Sandgem Town. Enter the house of Professor Rowan's assistant at the south end of town. This house will have a sign in front of it with the assistant's name displayed on it.
Speak to the little girl inside the house. She will tell you what Pokemon is swarming for the day. The Pokemon that is swarming is random every day, so visit every day until she tells you that Beldum is swarming.
Travel to Route 374.
Walk in the tall grass, initiating battles against random Pokemon until Beldum shows up. There is a 40 percent chance that the Pokemon you are fighting will be Beldum.
Use your Pokemon to weaken Beldum until his health is in the red. If possible, use any Pokemon moves that will put Beldum to sleep or paralyze him, as this will make him easier to capture.
Use any Poke Balls in your inventory to capture Beldum once he is weakened. As there is a random chance Beldum may break free, continue to use your Poke Balls until he is captured.