The D750 does not have a PC terminal (the default way to connect a flash. Also, the PC port is not a USB port for a computer. Rather it is a round connection in a round connection). In this case, you will need a hotshoe adapter in order to connect the cord.
http://www.paulcbuff.com/hsa.php Alternatively, you can get a radio remote system that allows you to trigger the flash without the camera being tethered by a cable. A basic set up would be a CST and CSR (the 2nd and 5th items):
http://www.paulcbuff.com/cybersync.php If you need something immediately, you can set the D750's pop up flash to manual and optically trigger the Alien Bee. You will need to set the pop up to manual so that the preflash does not fire and trigger the AB too early. You can also use a speedlight (i.e. SB700 or SB910) in manual mode to do the same. In either case, you will also likely want to dial the power down so that it does not impact your exposure. Specific settings will vary based on the device, but will be detailed in the operator's manual(s).