This is a great question, but the answer depends on how you are taking our classes:
With the free classes (http://free.startasl.com), slow motion is not available.
With the Start ASL Online Course (http://online.startasl.com), you can slow the videos by clicking on the minus (-) button that appears while hovering your cursor over the videos.
With the Offline Course (http://online.startasl.com) or Teacher's Curriculum (http://teachers.startasl.com), you can only slow the videos if you are using a video player on your computer that supports that ability. Two video players that offer this ability are Windows Media Player 12 (by right-clicking in the player and adjusting Enhancements > Play Speed Settings) and Quicktime Player (by pausing the video and using your arrow keys).