DotNetZip Library Error - The type or namespace name 'Ionic' could not be found

I try to add some Zip functionality of my website. I use DotNetZip from Codeplex. It works without any problem in my Development Machine (Windows XP). So I upload the files to the WebServer. Now I am getting the following error.
 
Compiler Error Message: CS0246: The type or namespace name 'Ionic' could not be found (are you missing a using directive or an assembly reference?)
How can I debug it. Please help me.
...