Assuming I understand what you mean:
If your UserControl
is in a library you can add this to you Toolbox using
Toolbox -> right click -> Choose Items -> Browse
Select your assembly with the UserControl
.
If the UserControl
is part of your project you only need to build the entire solution. After that, your UserControl
should appear in the toolbox.
In general, it is not possible to add a Control from Solution Explorer, only from the Toolbox.