First, the compatible Operating Systems and Browsers for Mac are Macintosh OS 10.5.7+ (intel-based) and Firefox3.6+ or Safari 4+.
http://www.microsoft.com/getsilverlight/get-started/install/default.aspx
If your system meet the minimal requirements and compatible browser, then please verify that you uninstall and reinstall Silverlight on your Mac follow the steps introuced in the following links:
Removing Silverlight plugins on Mac: http://www.microsoft.com/getsilverlight/get-started/install/removing-silverlight-mac.aspx
Uninstalling previous versions of Silverlight on Mac: http://www.microsoft.com/getsilverlight/get-started/install/uninstall-mac.aspx
Reinstall Silverlight on Mac: http://www.macupdate.com/app/mac/26623/silverlight
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