Check your plug-in manager and make sure that the right plugins are checked....

window-->Setting&Prefs-->Plug-in Manager

by default there is a 3dsimport.mll....but I haven't found an export one.....maybe someone else has more insight...


these are ones you should have though:

dwgTranslator.mll
vrml2export.mll
objExport.mll

that should get you on the right track....