Have you used the card yet to make an eBay purchase? Once you use the card to make a purchase any remaining amount is attached to the eBay account and should appear as a line item with a check box on the checkout page that can be applied to future orders.
If you have not used the card yet at all, here is what the eBay gift card FAQ page says:
https://pages.ebay.com/giftcard/faqs.html
