Have you been successful in controlling the Agilent device in LabVIEW? I cannot seem find the LabVIEW instrument driver for the Agilent E4991A. I am not sure how familiar you are with using third party products in LabVIEW, but we have instrument drivers for each product so that you can successfully communicate with the device.
With that being said, if you have an instrument driver for this specific device, could you please let us know where you found it?
Typically, there are examples in the instrument driver folder at this file location if you have the driver for the device:
C:\Program Files\National Instruments\LabVIEW 2010\instr.lib\Agilent E4991A\Examples
Additionally, if you do find the instrument driver, here is a KnowledgeBase article that will help you get the download into the correct folder.
I hope this helps and please let me know if you find that instrument driver.