When you perform a "Scan for Instruments" in Measurement & Automation Explorer, does it find your instrument? If not, have you tried to communicate with it from the Interactive Control window?
Here is a link to a document with more information on how to use Interactive Control:
Introduction to the Interactive ControlThis document also contains a link to some common errors and other troubleshooting techniques.
Lastly, there is another discussion forum regarding this same instrument titled "Need driver for ITC503." It contains some additional information and links to code for communicating with the instrument in LabVIEW.