Please verify that your reference to the WebControl project is set to the right location.

In the Solution Explorer, right-click on the DotNetNuke Project.  Choose Properties, When the dialog comes up expand the Common Properties folder (if not already expanded) and click on Reference Path.  On the right you will see a list.   It should read

[DNNPATH]\controls\SharpZipLib\bin\

[DNNPATH]\controls\DotNetNuke.WebControls\bin\

[DNNPATH]\controls\CountryListBox\bin\

[DNNPATH]\controls\DotNetNuke.WebUtility\bin\

Where [DNNPATH] represents the location where you have DotNetNuke installed.

If there are other paths in the list remove them, if some are missing add them.  When you compile, make sure you are using the BuildSupport project.  Right-click on it and choose Build.