Hi there,
The most likely explanation is that AutoCAD 2008 was installed after installing Jetstream v5. Exporters will only be installed if a matching program is found at the time of installation. If so, modify your v5 installation through Add/Remove Programs and select the AutoCAD 2008 exporter plugin.
If that is not the case, then you'll need to check that the appropriate files and registry entries are in place. Ensure that there is a file nwexport2008.arx on your machine and under the registry entries for AutoCAD you have a key of the following sort of form: HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\AutoCAD\RXX.X\ACAD-XXXX:XXX\Applications\Navisworks5. The strings under that key should point to that ARX file.
Those entries are what allow AutoCAD to identify what NWDOUT actually refers to.
I hope that helps,
Jeff
Senior Software Engineer
Navisworks
Autodesk Ltd.