Issue:
You try to add SharePoint WebControl to your custom web part. You don’t seem to see the web part under the Microsoft.SharePoint.WebControls Namespace.
The issue is that the WebControls do not work on Sandbox Solutions.

Resolution:
Follow these steps the change your existing project to turn the project off as Sandbox solution –
1. Click on the project and check the properties window or press F4.
2. On Properties you will change the value of Sandboxed Solution property to False.

Once the above changes are done now you can use the SharePoint Web Controls on your web part.

Happy Coding….