your problem might be that your object doesn't have a roadway lod. to create it rightclick on the lod window in o2 and click on 'new' in the context menu. then ricghtclick on the new lod (will be a res lod, inserted after your highest one), go to 'properties in the context menu and select 'RoadWay''.
on all faces in this lod soldiers can walk and vehicles drive.
for more info take a look at brsseb's tut on making a house:
link