In order to embed a twitter feed to your Prism Site you first need to create a widget on twitter.
Go here: https://twitter.com/settings/widgets
Click 'Create new', and run through the configuration settings. When you're done hit 'Create Widget'.
You'll then see a new box with HTML code in it:
Copy that code, and go to your Prism Site.
Click the '+' button found between the stripes and add a new HTML stripe:
Delete the code that's is currently in the HTML stripe, and paste the code you copied from twitter.