Use External Vouchers field in the offer set up to YES, then save the offer (screenshot). After, go to offer management to view the offers list, in the Actions dropdown you'll see "Load Vouchers". (This option is only there if the Use external vouchers field is set to yes) Click that and pop up will appear that will let you download a demo file and also upload your own file.
Be sure to use the exact file type, don't have any spaces in the file name, and ensure the headers are in place. Download the Demo file from the popup.