In the Birdseye Download wizard on basecamp software make sure in the dropdown products box that you have selected the correct country / product for the Birdseye Voucher that you have activated - e.g. if you have activated a Birdseye Select Great Britain Plus voucher make sure this is selected in the Products box.
Make sure you then click on the 'Where is my credit?' box and enter your Garmin user name and password that you used when activating your Birdseye Voucher to Activate your credit.