Using .3ds works fine with me, and it exports the texture mapping just fine. Make sure your texture filename is less than 8 characters long and that it is either .bmp, .dds, .tga or .pcx. Also make sure the texture file is in the same folder as your .3ds file, else MED will ask you to find it manually.

When you export a .3ds file from 3DS Max it should ask 'Save 3DS Max Texture coordinates' or something similar. Make sure you say 'Yes'.