In order to enable the Twitter log in for the users, you need to create a Twitter app and copy the app Consumer key and secret in the admin panel, on the Settings->Configuration Options page.
To do that, you have to log in on https://developer.twitter.com with your Twitter username and password and click on Apps at the top right corner -
and then on the Create an app button at the top right corner.
You'll see on the next screen the form for creating an app and you need to fill the fields there like the App name, enter a description and your website url.
On the same form, in the Callback URL field, please enter
https://www.yourdomain.com/index.php?mod=login-linkedin
(by replacing www.yourdomain.com with your domain name / the one on which the script is installed) and submit the form.
This will create the app and Twitter will redirect you to the app management page - please click on the second tab Keys and tokens and copy from there the Consumer API key and secret and paste them in the corresponding fields on the Configuration Options page and this will enable to log in for the users.