If "Create new account" is grayed-out for a new Eye-Fi card, Eye-Fi Center or Eye-Fi Helper probably does not recognize an Eye-Fi card is inserted. First, remove and reinsert your Eye-Fi card; If "Create new account" is still grayed-out, please see Card not Recognized by Computer for more assistance.
If you are running Eye-Fi Center version 3.3 or earlier, your card may already be activated. To determine if your Eye-Fi card is already activated, make sure that Eye-Fi Helper is running and plug your Eye-Fi card into the Eye-Fi 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 Eye-Fi Helper icon in either your Apple Menu Bar or Windows System Tray and you will see the MAC address of the Eye-Fi 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 Eye-Fi card, that card is not recognized by the computer.
If you see (registered), you may want to return the Eye-Fi card from the place of purchase and exchange it for a new one. If you purchased a used Eye-Fi card, please see Gifting or Buying a Used Eye-Fi Card for help activating it.