I created a space invader in magicavoxel, exported it into an obj and got a obj file, mtl file, and png file. When I load it into besiege, it doesn't work. Here's what it looks like: http://imgur.com/zGKCVPn
What block do you want to modify ?
If its the bomb you need to have the right named folder (Bomb) and rename the obj as Bomb.obj ant the png as bombnotex_png