I've not figured out the route or way but let me just say I'll reply when I can save the
day.. .. Hah hah..! lol..

Grab a hold of the vine as you have in the previous parts of this level. Press enter
multiple times to climb up almost to the very top. Turn so that you face the mushroom in
question. Swing back and forth as much as possible, and at the height of it, jump forward
onto the mushroom. You will be hurled upward, and just a mi***e bit of steering is needed
to get you onto the platform above the fine. Once here, you can make the jump across the
tree trunk, and exit onto an other part of the level. I hope this has been helpful.