ZBrushCentral

Material file incomplete?

Im using ZBrush for XNA Game Development. I export my models in obj format along with their materials as mtl, but the materials dont work in XNA. It might or might not be because the mtl file only has these lines:

File created by ZBrush

ToolType: PolyMesh3D

www.zbrush.com

newmtl defaultMat
map_Kd SomeBitmap.bmp

How can I solve this?