Ah! So you have a couple of issues: you need to know how much current it needs (solvable by giving it a supply with too much, as it will only take what it needs) and most importantly to know which pin on the barrel connector is positive. Usually (but not always) the center pin is positive. Connecting it up backwards will likely blow the device. Assuming the USB connector on the device is powered by the device, the ground pin on the USB connector (a USB breakout board would be the easy way to find that) should connect to the 5 Volt ground. An ohmmeter connected to USB ground then to the two pins of the barrel connector in turn should maybe identify which one is ground (making the other the 5V pin). If you have a 5 V supply and an ammeter, after determining the polarity of the power jack you could feed it 5 volts from the power supply and measure how much current it draws (check that the voltage remains 5 volts too, if the device draws too much current the supply voltage may drop as well). Then find a wall wart with at least twice that current capacity and regulated 5 volts and you should be away (either find one with an appropriate barrel jack or solder a new barrel jack on to the wall wart). Note that some or all of these steps may damage the device (if for instance it wants too much current and the input voltage drops and the device doesn't like under voltage), so only do it if you don't mind perhaps damaging the device. Hope this helps.
Peter Van Epp