Hopefully the droids.it USB board comes with a USB driver. This driver would likely work similar to the one we use with our XBIB-U-DEV USB development board.
What the driver does is create a USB serial port, through which your C++ program would then communicate serially to the USB board with XBee loaded on it.
Yes, the USB linked board creates a COM port on your PC. From there, you can create any application that you want to talk to that adapter by going through the serial port it has been assigned to by the OS (after installing the drivers). From here, everything else is standard RS232 programming, depending on whether or not the radios have AT or API firmware flashed in them. I hope this helps!