hello,
Is that assembly located either in your applications BIN directory, or in your servers GAC? You also need to make sure that the Infragistics2.WebUI.Shared.dll assembly is located in one of those two locations as well.
There are a number of help topics on deploying applications that use Infragistics tools in the help:
http://help.infragistics.com/Help/NetAdvantage/NET/2007.3/CLR2.0/html/Web_Deploying_Your_Application.html
Devin