The issue may be with how Cookies are handled in Safari. The machines we have tested on are set to "only allow cookies from sites I visit". Changing this setting to allow all cookies seems to allow the lesson to load properly.
It also appears you can remedy Safari short term by simply visiting the SoftChalk Cloud site before opening the lesson, (still having the setting on to "only allow cookies from sites I visit".")
If you have tried the above information and are still having issues please contact us at firstname.lastname@example.org