If you have recently installed the Atalasoft SDK and the controls are not showing in your Visual Studio toolbox the most common cause is having a project targeting Client Profile. We do not currently support Client Profile, but this may change in the future.
First, if your project is targeting .Net 3.5 or .Net 4.0 make sure you are not in a Client Profile. In C# it is under your project properties, in the "Application" tab. There is a drop down titled "Target framework:", make sure it doesn't have "Client Profile" in the name. If it does, change it to the equivalent .Net target without "Client Profile" and the controls should appear if they were registered on installation.
If that resolves the issue, yay! If not, the next step is to try manually adding the controls to your toolbox.
Bring up your Visual Studio toolbox and right-click on it. There should be an option saying "Choose Items...", click it. At this point it may take a few moments to load and it will show you a dialog box with all the references available that do not have UI controls in them. This is normal and expected, simply dismiss this dialog. Then sort by Namespace or Assembly name, and select the controls within the Atalasoft SDK (all start with Atalasoft.) Once you have done that click "OK".