To add a custom control to VisualStudio project:
1) Go to “Toolbox” tab;
2) Right click on it and click “Choose Items…”;
3) In the “Choose Toolbox Items” window click on the “Browse…” button.
Navigate to the location where your Dynamic Link Library (DLL) file is.
Click “OK”. Now your custom control is included into the Visual Studio toolbox.
For example of how to create custom control with C# and Visual Studio read this article: