Hello SamiL,
I believe the USB-6251 pinout has a +5V pin, and depending on the current requirements of your sensor you can probably use this to provide the bias voltage to your sensor. (Page 11 of the <a href="http://www.ni.com/pdf/manuals/371291h.pdf" target="_blank"> NI 625X Specifications</a> says the USB-6251 can provide up to 1A on the +5V pin.)
For the V+ 3.3 V pin, I would have to consult your sensor's manual. Is this a 3.3V input or output? It it?s an output, you could measure it with another analog input channel; if it?s an input then you can either use the +5V pin again, or use an analog output to provide 3.3 volts if the current requirement is less than 5mA. Since you have bias terminal it makes me think the V+ pin is probably an output, but to be sure you'd need to check the sensor's manual.
For the connection of the GND, Ref Out, and Signal Out I would refer you to table one of this article: <a href="http://zone.ni.com/devzone/cda/tut/p/id/3344" target="_blank"> Field Wiring and Noise Considerations for Analog Signals</a> basically you'll need to find out if your sensor's output voltage is ground referenced or floating. Then you can choose to measure differentially or single ended based upon your channel requirements. It?s important to note that if your sensor is ground referenced you don't want to connect the sensor's GND to AI GND because this can cause a ground current loop between the devices. However, if you're powering the sensor using the +5 volt pin on the USB-6251, then even if your sensor is ground referenced you should be fine because the card and sensor already share the same ground reference.
I hope this helps, and if you still have questions it would be helpful if you can post more information on your sensor.
Cheers,