If you have activated the Authentication extension, you have the option to allow your users to log into your app with their Twitter account (native apps only).
You already set a Twitter section, it means that you have created your Twitter application:
1. Go to Settings > Registration settings > Registration process :: Connection/Subscription tab
2. Under "External Services" turn the "Authorize Twitter connection" button to ON.
3. Paste the Consumer Key and Consumer Secret in the appropriate fields.
You don't have a Twitter application yet:
1. Follow the instructions given in this online help to create a Twitter app
2. Copy your Consumer Key and Consumer Secret
3. Paste them in the menu Settings > Registration settings > Registration process :: Connection/Subscription tab
Note: To set up the login via Twitter, the Access Token and Access Token Secret are not necessary, follow up instructions up to step 3 included, of the Twitter online help .