Files from Nick's Chassis pack dont need to be imported. They are blend files and can be opened by blender. Basically File...... Open...... Find your directory in which they are and then find the blend you want to open. Or find the directory and click on the blend file to open blender with it.
Now if you want to import and allready exported chassis..... Say HooliganChass.mesh you need to first convert this to an mesh.xml file In your program files, Rigs of Rods 37.xxx there is a ogre xml converter, drag the file over top of this and let it run. It will produce a .xml file. This then allows you to import it via the orge xml importer.
Stephen Hopkins's post in Export/Import was marked as the answer