OK, here we go. Sorry for the delay, I had to download and install Gmax, then try and figure out how to use it
A (very) basic run though of texturing a box. I'm using 2 materials, one for the top and another for the sides which has been split into sections.
In Gmax, I've created a simple box, and set up 2 materials. Make sure you don't use multi-material! Remember, it's one material per object. I'm just using the standard material as I don't have the FSX SDK installed with Gmax.
Convert the box into editable poly, or editable mesh, go into poly sub-mode and select the top face. Click "detach" and give it a new name.
Select the new object & give it the material for the top. Apply a UVW mapping modifier. You'll probably need to play around with the alignment buttons to get the orientation correct.
Back to the original box. Again in poly sub-object mode, select and detach the sides on at a time until you've only one side left. You can delete the bottom face if you like as that won't be seen anyway.
Select the object that is the first side. Apply the side material and a UVW mapping modifier.
Apply an Unwrap UVW modifier & move, rotate and scale the points until you're happy that it's lined up correctly.
Repeat for the other sides.
When all the sides are done, add an edit mesh modifier to one of the sides, and attach the other sides to it.
You can now weld the adjoining verticies. Remember, the top has a separate material, so you can't join that to the side object.
You can add an Unwrap modifier if you with to tweak the alignment further.
That's just one way to do it, there are dozens of others! I tend to do things this way as it make it less confusing. If you've got a complex object with loads of mesh select and UVW modifiers on it, it can be a pain to keep track of what is what.
Hope this has been helpful.