Find the location where AjaxControlToolkit is installed and under References, choose "Add a Reference" and select the AjaxControlToolkit.dll.
Once you do that and build the project the error should be gone. Assuming the reference is added with Copy Local is true it will also appear in your bin folder, so when deploying the contents of bin to any server it will still work and no special installation is necessary for AjaxControlToolkit.
In addition you might not want todo this on every Page or UserControl, so maybe better to register it in the web.config once (use a better prefix "ajax" instead of "cc1"):
<configuration>
<system.web>
<pages>
<controls>
<add tagPrefix="ajax" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
</controls>
</pages>
</system.web>
</configuration>