Add the form to VBA, add the field to VBA, then copy in this code. It's fully commented
Private Sub Window_BeforeOpen(OpenVisible As Boolean)
Dim t As String
'get the current time
t = Time()
'get rid of the colons
t = Replace(t, ":", "")
'if it's AM, get rid of the AM text
t = Replace(t, " AM", "")
'PM is more difficult
If Right(t, 2) = "PM" Then
'get rid of the PM text
t = Replace(t, " PM", "")
'move the time up 12 hours (military style)
t = t + 120000
End If
'for times before 10 AM, this result at this point will be 5 characters
'add a sixth char
t = Right("0" + t, 6)
'all set
Me.ADCTime.Value = t
End Sub