You'll be wasting your time with normal or bump maps for leaves/branches. Aside from the fact that you'd not notice the difference, it's not compatible with alpha transparency.
But to answer your question, no, you don't need Max/GMax. The flag can be set in the ASCII, as JC said, or in MDLEdit you can just go to Edit -> Textures and tick the box next to the one you want to use a bump/normal map. Note that if using a normal map it must be converted to a TPC. A TGA will cause the game to throw an error on loading.