Hi Thomas,
you can check whether your VISA driver is installed correct, if you take the MAX (Measurement & Automation Explorer) and open the VISA testpanel to do the things you want to do in your program manually with the testpanel. Go to 'my system'-> 'devices & interfaces' -> 'serial & parallel' and open the wanted port to do the test. You should also check if have installed the latest VISA driver. You can do so at 'my system' -> 'software' -> 'NI-VISA'. The latest version is 4.2 you load it from
this page.
I hope this answered your question, if you got any other question feel free to post it here.
TomBaum