it's hard to catch a munchlax, first you need to buy a honey or the cheaper way catch a combee and walk around awhile then you'll see your combee will be holding a honey, then slather the honey on a sweet smelling tree,then come back for it later i think it's after 12 hours then come back
but remember it's hard to catch a munchlax, it it the rarest of pokemon who you can catch via this way