Its a harder process to explain than to do.

Basically you need a 'Skin' material which will be a clone of your diffuse wing material with the Skin box checked.

Then you need a hierarchy of bones that correspond to where you have vertex loops around the Wing see the example of how the hierarchy is structured on on the DC3.

The lowest bones in the hierarchy must be anchored at the base, i.e have animation tags but not move, and be at the bottom of the hierarchy of the main model.

Then work out from the wing root animating the required upward rotation of the flex on the bones only. Don't forget to move the animation slider back to zero when done.

Once done apply a skin modifier to the wing mesh and add the bones to the modifier. Make sure that all vertices at the wing root area (in the DC3 example this means the whole fuselage!) are 'Weighted' and influenced by the scope of the lowest bone. Then again working from the wing root work outward and make sure the vertices that you want to be affected are being influenced by the scope of the corresponding bone.

That's basically the procedure you can see on the Douglas example how the flaps and ailerons are included in the structure but the same principles of this procedure apply.

Then you'll need to add the wing flex animation tags (the default code or your own custom code) so the flex you see in Max (and now gMax...  ) happens in sim.

It does warrant a proper tutorial really as this description doesn't do it justice but I hope it helps.

Jon.