If you open a text resource or type in some text in the text tab on the textivate edit screen, you'll see a new "Trapdoor" button. Click this to add a Trapdoor activity to your resource.
A Trapdoor activity is a multiple choice activity where students click their way through the options to (re-)build the text. It is called Trapdoor because if they get a section wrong, they fall through the trapdoor and they have to start again.