You may just need to recompile the code, in order to register the assembly on your box. Otherwise this error typically occurs from improper namespace usage between your assembly and the web page referencing the namespace used by your application.
You need to make sure your namespace matches the inherits attribute in the global.asax application codebehind tag. You have to open the file manually since visual studio doesn't really let you open it directly.
c#
<%@ Application Codebehind="Global.asax.cs" Inherits="__NAMESPACE__" %>
vb.net
<%@ Application Codebehind="Global.asax.vb" Inherits="__NAMESPACE__" %>
Change the attribute, so your inherits (namespace) matches your project namespace and recompile. this is a pretty well known issue, but keep in mind it's also *case sensitive* as well.