Instructions
Go to Floaroma Town by taking Route 205 all the way to the west; the town is on the west coast of the map. Head to Floaroma Meadow by following the dirt road running through Floaroma Town all the way to the north. Purchase some sweet honey from the vendor standing in Floaroma Meadow. Sweet honey costs 100 Pokedollars.
Stand next to the yellow tree that is close to the honey vendor. Press the "A" button, and then select the sweet honey from the pop-up menu to slather the honey on the tree.
Wait 12 hours, and then come back to the yellow tree in Floaroma Meadow. Look to see if the yellow tree is shaking. If the tree is not shaking, wait another 24 hours and repeat the previous steps until Muchlax appears.
Press the "A" button when you are next to the tree to fight the Pokemon in it. There is a 1 percent chance it will be Munchlax. If the Pokemon is not Munchlax, wait another 24 hours and repeat the previous steps until Munchlax appears.
Attack Munchlax with Normal-, Grass- or Water-type attacks, such as Tackle or Bubble. Munchlax has high Fire and Ice damage resistances, so avoid using those types of attacks. When you've reduced Munchlax's health to under 25 percent, throw a Poke Ball at it to capture the Pokemon.