I would also advise using CatRig instead of the pretty old IK solvers that ship with Max. CatRig is a hybrid FK/IK system and also has a layers features that would work well for turning on/off different anims like your landing gear.
The tutorial you linked has everything you need:fk/ik/auto-pistons. If it didn't work the first time then start again at the very start and keep at it until it works. I will try and view your file if I get the time.
Ok I solved by linking theactual joints not to the attached platform but to the one above or under . Now its working as intended, but I have no idea how to actually connect in a fluid manner, I tried all sort of IK controller options but they always block my rotations or other stuff, the idea is that this thing shoudl now…