First, make sure Eye-Fi Center is closed, and all associated helper background services are shut down. Do this by checking your system tray (Windows) or your menu bar (Mac).
Look for ~/Library/Eye-Fi/Settings.xml
NOTE: If you can't find your ~/Library folder, it may be hidden from view in Finder. To fix this, open up terminal, and at the command prompt, type: chflags nohidden ~/Library
Look for C:\Users\[user]\AppData\Roaming\Eye-Fi\Settings.xml - Where [user] – is your current login name.
C:\Documents and Settings\[user]\Application Data\Eye-Fi\Settings.xml
Right-click on that file, and choose edit. If you don't have that option, open that file in your favorite text editor like Notepad or Text Edit. DO NOT open this file in a word processor like Word, Pages, Open Office, etc.
Now, launch Eye-Fi Center and again configure your Eye-Fi card, as outlined in the following Knowledge Base article:
If you're having difficulty with your Eye-Fi card upload key not being shown, follow the steps in this guide: