Tangela is unobtainable in Diamond and Pearl. You can only get it by Pal Park from FireRed or LeafGreen. And Munchlax only has a 1% encounter rate in Honey Trees. If you keep spreading honey everyday, you should eventually find it.
Here's a youtube video of what the tree with a Munchlax in it should look like:
http://www.youtube.com/watch?v=NqLtl8K8T4Q
It's more stiff and shakes quickly.