Creating a loop to count number of checkboxes Powerpoint

I have slideshow with an action button that allows the viewer to advance the slide ONLY if all the checkboxes on the page are filled in, Some slides have between 1 and 20 checkboxes. I want to create a for loop that will count the number of control check boxes on each slide so that my action button knows how many checkboxes it must confirm have been clicked.

I ran across this bit of code

Private Sub CommandButton1_Click()
Dim ctl As Control
Dim j As Long
For Each ctl In Me.Controls
If TypeOf ctl Is MSForms.CheckBox Then
 j = j + 1
End If
Next
Unload Me
End Sub

How do I get this to work in Powerpoint??? I'm struggling to find the correct checkbox syntax