Cannot Add Custom Controls to Toolbox

Hello - I am trying to add custom controls to the toolbox but when I
right-click on the toolbox nothing happens.
I am working with a form region that I am attempting to modify with
additional controls
I've seen that other people have had this problem but have not found a
solution. One suggestion was to open the toolbox in Outlook VBA and add the
controls there but that does not work either. Nor does running the office
diagnostic or restarting the computer. Can someone please help me with this
problem!!
Thanks!