iWeb Videos and Music Play on iPad but not MacBook

Hi,

I have created a website on iWeb and everything plays perfectly on my iWeb. When I launch the site on my MacBook and my husband's MacBook Pro, however, the videos and audio clips do not come up, and they just are blank with a gray "Plug-In Failure" in place of the video. But I can play them just fine on my iPad and iPhone. We are both running Lion--is that the problem? Please help!

Thank you,

Adrienne