Add the following entries to the compilation node in the web.config file.
<compilation defaultLanguage="c#" debug="true">
<assemblies>
<add assembly="Infragistics2.Shared.v8.1, Version=8.1.20081.1000,
Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics2.Win.UltraWinTree.v8.1, Version=8.1.20081.1000,
Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics2.WebUI.UltraWebTab.v8.1, Version=8.1.20081.1000,
Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics2.WebUI.UltraWebGrid.v8.1, Version=8.1.20081.1000,
Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics2.WebUI.Misc.v8.1, Version=8.1.20081.1000,
Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics2.WebUI.Shared.v8.1, Version=8.1.20081.1000,
Culture=neutral, PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="Infragistics2.WebUI.UltraWebNavigator.v8.1,
Version=8.1.20081.1000, Culture=neutral,
PublicKeyToken=7dd5c3163f2cd0cb"/>
<add assembly="AjaxControlToolkit, Version=3.0.30512.20315, Culture=neutral,
PublicKeyToken=28f01b0e84b6d53e" />
</assemblies>
</compilation>