If "Create new account" is grayed-out for a new Eyefi card, Eyefi Center or Eyefi Helper probably does not recognize an Eyefi card is inserted. First, remove and reinsert your Eyefi card; If "Create new account" is still grayed-out, please see Card not Recognized by Computer for more assistance.
If you are running Eyefi Center version 3.3 or earlier, your card may already be activated. To determine if your Eyefi card is already activated, make sure that Eyefi Helper is running and plug your Eyefi card into the Eyefi USB card reader; please also make sure that the USB card reader is plugged directly into a USB port on your computer rather than a USB hub.
Click once on the Eyefi Helper icon in either your Apple Menu Bar or Windows System Tray and you will see the MAC address of the Eyefi card; the MAC address begins with 00-18-56, and is followed six more numbers and (registered) or (unregistered). If you do not see the MAC address of your Eyefi card, that card is not recognized by the computer.
If you see (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, please see Gifting or Buying a Used Eyefi Card for help activating it.