"Create new account" being grayed out for a new Eyefi card usually means Eyefi Center or Eyefi Helper does not recognize that an Eyefi card is inserted. Remove and reinsert your Eyefi card; if "Create new account" is still grayed out, please refer to What do I do if the Eyefi Center doesn't recognize my card? for more assistance.
If you are running Eyefi Center version 3.3 or earlier, your card may already be activated. To determine if your Eyefii card is already activated, verify that the Eyefi Helper is running on your PC or Mac, plug your Eyefi card into the USB card reader. Next, plug the USB card reader into a USB port directly on your computer instead of using a USB hub.
Click once on the Eyefi Helper icon in either your Apple Menu Bar or Windows System Tray and you will see your Eyefi card's MAC address, which begins with 00-18-56. There are six more numbers followed by (registered) or (unregistered). If you do not see the MAC address of your card, this means it's not recognized by the computer. See What do I do if the Eyefi Center doesn't recognize my card? for more assistance.
If the Eyefi Helper shows (registered), you may want to return the Eyefi card from the place of purchase and exchange it for a new one. If you purchased a used Eyefi card, see Gifting or buying a used Eyefi card for help on activating the card.