If your are using the Bitly API, you should no longer be using your API key. Instead, you will want to grab your Oauth Access Token, which you can find in ‘Advanced Settings’ below ‘API Support’.
Some third party apps, like Tweetdeck, still ask for your username and legacy API key. To find your username and API key, navigate to the menu button and select ‘Settings’ from the drop down. On the ‘Advanced Settings’ tab locate ‘API Support’ where you will find your API key. To reset your API key, select ‘Reset API Key’ located below your API Key in ‘API Support’.