WARNING Obsolete:

Atalasoft newer versions no longer support Silverlight web controls. Supports ability to assist in keeping existing Silverlight deployments working may be limited. No further fixes/updates will be issued for our Silverlight assemblies/controls

Main Article

If this is occurring, please verify that you have performed the two following tasks:

1. Added a SilverlightWebHandler or the SilverlightAnnotationWebHandler to the hosting aspx page
2. Modified the Web.Config file to include two keys:

<configuration>

<appSettings>
    <add key="AtalasoftSilverlight_Cache" value="<Put Cache Folder Name Here>"/>
    <add key="AtalasoftSilverlight_CacheLifeTime" value="10"/>
</appSettings>

...

</configuration>

Original Article:
Q10326 - FAQ: I have a SilverLight viewer and when I call open it loads indefinitely