When editing the user form in VBA, be sure to have the Toolbox displayed by going to the View menu and selecting 'Toolbox'.  Then do the following:

1. Right click anywhere on the Controls tab of the Toolbox and select 'Additional Controls'.
2. Scroll through the available controls and check the box next to the desired control.
3. Click OK.

The additional control should now appear on the Controls tab of the Toolbox.