Quote from Jedo;1243910:
I purchased an ace character card so I could rebirth as merlin. However, I went in to rebirth and there is no option to use the card at all.
You're behind the times. :p Character cards are no longer used for rebirthing. Instead, you'll want to disenchant the card, which will break it into pon (which are used for selecting hero talents now) and any applicable gear/items/titles that would have come with the card. It should cost about 39 pon to select a hero talent, give or take a few pon.