When a lesson is launched, you will see the message pictured below. You can click the brown link to allow the popup and the lesson will immediately launch. If you would like to "always allow popus from this site, see directions immediately below.
When you click the brown link featured above, you'll notice a little icon in the top right of your browser window. See icon pictured below. Click this icon to open additional options, one of them being to "always allow popups from the specific URL". Choose "Always" and click the done button.