Hmmm actually I think i used the Learning Pool one :

Go to https://github.com/LearningPool/adapt-contrib-responsiveIframe

Use the download Zip button on the right.

Go to the menu of the Authoring Tool > Plugin management

Click Upload Plugin

Navigate to your downloaded zip and upload this file

If successful you can pick Responsive iFrame as a new component in a block.

Hope that helps!

Helen