To start the PartViewer you have to specify the location of your LDraw Parts Library on the command line.
For example if your operating system is Linux and the path of your LDraw Parts Library is /usr/local/share/ldraw you may start the Part Viewer by typingjava -Dldraw.dir=/usr/local/share/ldraw ldrawloader.jar
Windows users that have installed the LDraw Part Library at c:\ldraw may typejava -Dldraw.dir=c:\ldraw ldrawloader.jar
If you have everything installed correct you should see the part 3001.dat (which is a 2x4 lego brick). If you want to view a custom file you can specify it at the command line as last argument, like:java -Dldraw.dir=/usr/local/share/ldraw ldrawloader.jar /usr/local/share/ldraw/parts/3005.dat
If you have trouble setting up everything correct you can set up a support request, so we may help you.