Google Merchant Center is launched to replace Google Base for submitting Product-type items to Google. The Merchant Center interface is very similar to the Google Base interface, Sign in to the Merchant Center using the same Google Account information you did for Google Base, and your existing data feeds, items, FTP settings, etc. will be there for you. For any non-product items you've submitted to Google Base, those’ll remain there and you can continue using Google Base to manage those items and settings. If you used the same account to submit both product and non-product items, you'll need to go into Google Base and reset your FTP settings.
More info: Merchant Center