Eyefi recently launched a new Mobi card; the 10 digit code is for customers who have Mobi cards. If you have an Eyefi X2 card, you do not require an activation code. Check to see which Eyefi card you're using:
You can find additional information on card setup at the Eyefi card support site.
When you launch the app on your mobile device there will be a message near the bottom that says "Already have an Eyefi account?" and a Sign In button.
When you tap Sign In, the app will ask you for the user name and password you created when you set up your card in Eyefi Center on your computer.
You can then sign in and pair your card with your device.