Problem using driver with the 6031A Agilent power supply

Hello and goodmorning,

I was trying to develop my first cvi application in wich i need to use a power supply (model Agilent 6031A). I first use direct GPIB command and i manage to make it work but with some trouble, due to the fact that sometimes the commands are missed.

So i have downloaded the driver, from the NI site, and i was trying to implementing it in my code. I found a problem with the very first instruction, the init one. Below are my code:

ViPSession InstrumentH;  
...
...
...
Appoggio=hp6xxxa_init ("GPIB0::5::INSTR", VI_ON, VI_ON, 1, &InstrumentH);

 After the above code was executed i found in "Appoggio" the value "-1073807339"  and InstrumentH is equal to NULL.

Any idea and advice would be very appreciated.

P.S.

There is a way, like in Labview, to find the explanation of the Error Code?