In order to be able to access your GPIB device using VISA you need to use a supported driver, which in this case would be:
National Instruments NI-VISA version 3.0 (or higher)
Reference:
Instrument Control Toolbox - Supported Hardware - VISA
<http://www.mathworks.com/products/instrument/supportedio13784.html>
For more information on VISA see:
VISA Overview
<http://www.mathworks.com/help/instrument/visa-overview.html>
If for some reason you cannot upgrade your VISA driver, then you can also access your GPIB device directly from MATLAB as it possesses this capability. For more information see:
Controlling Instruments Using GPIB
<http://www.mathworks.com/access/helpdesk/help/toolbox/instrument/f13-5310.html>
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