I managed to finally find one on Diamond about... 3 years ago, I think, on the route north-east of Solaceon Town before reaching Veilstone City. Up to then, I thought that I would be within the doomed majority to never, ever find one in the Gen 4 games! Munchlax was also assigned to me as a Scramble Challenge team member on one of my earlier playthroughs, but due to how elusive they are, I ended up seeing out the game with 5 team members.
It's incredible just how rare Game Freak made this fella to encounter, considering that he was the poster boy of Gen 4 for a while until Lucario stole his thunder. Before HGSS arrived, you had to either Pal Park a Snorlax from Gen 3 and breed it, or bank on the 1% chance of finding one on a Honey Tree.
Nope, nada, zip, not even a nibble. The rarest Mon I've found via honey trees is Heracross, but other than that, I've had zilch luck in any and all attempts to find Munchlax. So I just bred a Snorlax in the end.
Never seen a munchlax up those honey trees in my life! Didn’t even know you could get munchlax ( and at that rate too!) until recently as I thought the whole honey tree thing was a tad convoluted as a child.
I've only played Sinnoh games within the past 3 or 4 years, and all the attempts I've made in them have failed to see a Munchlax. The luckiest I've been when I actually came back to find a tree shaking was in my Nuzlocke where I got a female Combee.