I don't want to spoil anything, but I do wish to spare people who know this scenario the toil of searching though every corner of every level to find the entry point. So, I'll be ... obtuse, and give only another supplementary hint.
The start of the Tycho Plank is well into the game; it does not branch where the other two branch. So if you get "that far", no, you haven't missed it. To elaborate on one of the previous hints given, Durandal will be preparing to do something mortally unpleasant to Tycho, and will tell you about this in the exit terminal of a level. Knowing this, you can turn around and use this same dirty trick on Durandal.
It is NOT entered through some secret terminal, it is NOT entered through some bizarre gymnastic gyrations from one platform to another. It's very straightforward, right out in the open; just a matter of the "stupid monkey" (a cue from "D" that you've initiated the plank) going for the wrong hole.
"Gosh, I think that would fit in there. Why not?" Oops.
-smoochies,
Lysander
