When creating ActiveX components, an easy way to test them is to add a module to the project (see the code below). In this module you can write methods that will instantiate your component and then call the methods.
To make sure nothing was broken, you can keep your unit testing code with the project and call these functions when you have to edit your code.
Public Sub TestAddition()
Dim oAdd As cAddition
Set oAdd = New cAddition
'if the result is not 8 then something is not
Debug.Assert oAdd.AddNumbers(5, 3) = 8
Set oAdd = Nothing
Now all you have to do is type TestAddition in the Immediate window.