This is a Lightstream Labs feature. For info on enabling Labs features please visit this article
To add your StreamJar overlay, first visit the StreamJar website to copy the URL of the overlay you wish to add.
Once you have this copied, return to your Lightstream project, click the “Add Layer” button, and then click on "Alerts". Afterwards, you'll want to select StreamJar and then paste the URL you copied earlier in for your layer.