When using VS 2010 and have Crystal Report 13 installed as well as Crystal Reports 10, when you drag the crystal report viewer control onto your webform it registers the wrong assembly. It registers the following...
<%@ Register Assembly="CrystalDecisions.Web, Version=10.5.3700.0, Culture=neutral, PublicKeyToken=692fbea5521e1304"
Namespace="CrystalDecisions.Web" TagPrefix="CR" %>
When you correct the version to 13.0.2000.0, it works.