This can be a bit tricky. It is really related to Visual Studio and there are surely differences in the various versions.
I've struggled with this in the past also and it can drive you crazy.
Anyways, the easiest way that will probably work is to
. open your target project
. open the form where you want to use the control. (if you don't open a form then the toolbox item in Visual Studio doesn't normally appear)
. open your Toolbox (the place where you choose controls that you'll drag and drop onto a form).
4. Have you ever noticed these are grouped into collapsible tabs? well, go to the tab that says general...and go below it and right-click and a menu will appear that looks something like this --> https://i.stack.imgur.com/zwUqR.png
5. you want to select the [Choose items...] menu item.
When you select that then another dialog box will appear.
This is where you will choose the control that you want Visual Studio to know about.
6. click the browse button and browse to the location where you have the dll that contains the control and select it.
This will add it to the General list and will register it with VStudio and _probably_ solve the problem.
If you have a UserControl defined within your project you will have something like I have with a TextArea.cs (mine is a special textbox).
If you have that, then
1. go under Solution Explorer
2. navigate to your file (mine's TextArea.cs) and drag it over to the General tab and drop it on the Toolbox General tab.
3. This will create a reference to the control and register it so Visual Studio will know how to render it.
Either or both of these may require a VSTudio restart.