User Controls not showing up in the toolbox
Solution 1:
Check this setting:
Tools > Options > Windows Forms Designer > General : AutoToolboxPopulate
It should be set to True for this to work.
Solution 2:
Build your project to make sure it compiles.
With the form that you want your user control on, open the toolbox, right click and select "choose items"
Browse to your .exe or dll that you compiled in step 1.
make sure that your user control has a tick next to it, press OK.
Your user control should appear in the toolbox, so drag it onto your form.
This is adapted from Calanus's answer to a similar question.