Well you, have to map the bitmap around the model in your 3d modeling program (i.e. the one you built it in). If you don't have, you could try milkshape or gmax to start with. Then in most good 3d modeling programs you can open a texture and paste bits onto the sides of your cube. However if it is a complex model, like in darkmatter, its harder to map a bitmap around it, so you can use an unwrap uvw function which does it automatically. Hence why they are all jumbled up in the bitmap.
Mapping textures to object can be very difficult, and I sugest learning the basics of modeling an object first.
