Things You'll Need
Instructions
Complete the main story of the game: Beldum can only be captured as part of a special "swarming" feature that does not activate until the game is completed.
Fly, walk, ride or otherwise travel to Sandgem Town. In the house south of the laboratory, a little girl will tell you which Pokémon is swarming today, as well as the Route it can be found on. If the girl mentions Beldum, continue to step 2. If not, have patience; the swarming Pokémon changes every day, and eventually Beldum will come around again.
Travel to either Solaceon Town, Snowpoint City or Pastoria City.
Locate the PokeMart, a building characterised by its blue roof. Talk to the vendor inside: if it is daytime, purchase at least 20 Ultra Balls. If it is nighttime, purchase a similar number of Dusk Balls instead.
Travel to the Resort Area in the northeast corner of the world map, and head north out of the town onto Route 228. Your arrival in the correct area will be heralded by the appearance of an endless sandstorm.
Find a Beldum in any patch of grass along this route. This will not be difficult as the Pokemon has a 40% appearance rate during swarms.
Reduce Beldum's HP bar to the red by means of attacks from your own Pokemon.
Throw Pokeballs until the Pokemon is captured. If it is daylight, throw Ultra Balls, as they are the most efficient means of capturing the Pokemon. If it is nighttime, throw Dusk Balls, as they are more effective than Ultra Balls, but only during the evening hours.