Put the collision mesh on the 2nd layer. Also, which version of the exporter are you using?
If you are using v0.97b3 then do the following:
1. Open the exporter
2. Locate the tab that says detail levels
2.1. Note "Detail1" and the 20 boxes.
2.2. Click on the 1st box
3. Locate the the "Add" button
3.1. Change "visible detail level" to "collision detail level"
3.2. Press "Add"
3.3. Click the 2nd box
4. Export
Also, turn off "double sided" before exporting. And fix the normals and the overlapping faces too. While these are not game-breaking faults, you should try making your meshes tidy, for a better performance and aesthetic appeal.