You will need to register your new Kindle with your existing Kindle account:
Step 1 - Turn your second Kindle on and press the "Home" button.
Step 2 - Press the "Menu" button and select "Settings."
Step 3 - Select "Deregister," followed by the "Deregister" button in the dialog box that appears. The second Kindle is unlinked from the Amazon.com account it is currently registered to.
Step 4 - Press the "Menu" button and select "Register."
Step 5 - Enter the email address and password ***** the Amazon.com account used by your first Kindle device, and then click the "Register" button.
Step 6 - Launch the Web browser on your computer and navigate to the Amazon.com homepage. Click the "Sign In" button and enter the same email address and password ***** to register your second Kindle. You are logged in to your Amazon.com account.
Step 7 - Click the "Manage Your Kindle" button in the "Kindle Support" section. The settings section for your Kindle devices is displayed.
Step 8 - Click the "Personal Document Settings" link in the "Your Kindle Account" menu.
Step 9 - Click the "Edit" link to the right of your second Kindle's name in the "Send-to-Kindle E-Mail Settings" section. Change the "@kindle.com" email address to match that of your first Kindle and click the "Update" button. Your second Kindle is now registered to the same email addresses as your first Kindle :)