what version of blender are you using?
add-on 0.3.0 works only with blender version is not higher than 2.61, otherwise it will work in unexpected ways. Recommended to use official builds
r44117 (2.61.4) and script 0.3.0. export fine but only scene without any geometry
r39307 (2.59.0) and script 0.3.0. works fine but sometime crash after export but render fine
On some previously made materials, when I open the Maxwell Material editor in Blender, they show up fine. But on some other materials, the material editor through Blender, shows no material. It just opens up a blank untilted material.
Edit:
I found that if I browse to the material using the browse button instead of the folder button to locate my materials, they material editor opens them up just fine.
Is it possible you can do a tutorial on how to set up a simple fur/ grass strand scene and export the scene to Maxwell Render. I tried setting up a simple scene but it is not working. I set up the strand particles and textures made sure it rendered correctly in blender. Then made a maxwell material per the directions on the maxwell site for srtand materials. But the scene will not export. Blender does not crash. it just will not export.