There are no components in DLL that can be placed on toolbox

In some cases you can solve this problem with drag and drop:

• open Windows explorer and navigate to the DLL
• drag the DLL and drop it on Visual Studio in the Toolbox, exactly where you want your components to appear

Sometimes drag and drop does not work also. In this case you can

• open Windows explorer and navigate to the DLL
• copy the dll file and paste it directly on the toolbox tab you wish to place the controls.

Have you checked the visibility of the custom classes?

If you haven't declared them as public they will default to internal and hence aren't exposed to Visual Studio for use.