I ran into the same problem: when I dragged the DLL onto the webform it just created a link to the DLL. Here is what worked for me:
Put the following on line two of your aspx file (right below the page directive)
<%@ Register TagPrefix="asp" Assembly="ExportToExcel" Namespace="KrishLabs.Web.Controls" %>
You will need to create a Gridview on the same page and bind data to it if you haven't already done so on your ASPX file. Then add at this line into your ASPX file where you would like the button to show up:
<asp:ExportToExcel runat="server" ID="ExportToExcel1" GridViewID="gvYourGridViewName" ExportFileName="ExportFileName.xls" Text="Export to Excel" IncludeTimeStamp="true" />
Be sure to change the GridViewID property to the ID of your GridView. Hope this helps and save some time!
PS - I discovered the namespace of the DLL using ildasm.exe which is included in the Visual Studio (specially SDK) install.