1. I think the target file and destination should be:
"j:\fac\2010.pdf#nameddest=Box: 42046"
...notice the change to a "#" symbol.
2. Do you know that that the named destination exists? I always troubleshoot these kinds of issues by going back to basics, plain vanilla. Try executing a complete command from a command line. Verify that works, then you should be able to know exactly how the Shell cmd should be built in your app.