I found this while I was looking !
Add the control to the forms design Toolbox.
Drag the control from the Toolbox to the custom form page.
Use the Value tab of the control's Properties dialog to bind the control to the appropriate date/time field.
Set the date/time format. (ActiveCalendar seems to require a format like "10 August, 1999" for the binding to work properly.)
Add code to the form's Item_Open event handler to maintain the proper size of the control.
(These controls tend to resize themselves.
Finding the right values for the code may take some trial and error.) Here's an example:
Function Item_Open()
Set oInspector = Item.GetInspector
Set oPage = oInspector.ModifiedFormPages("P.2"

' Error handling in case control is not
'available on machine
On Error Resume Next
Set oControl = oPage.Controls("AC20"

' Height 16 is equal to height 21 defined
' in Outlook property dialog
oControl.Height = 16
If err <> 0 Then
Msgbox "This form requires an additional " & _
"component that does not seem to " & _
"be available on your machine." & _
vbCrLf & "Please install " & _
"ActiveCalendar from the library."
err.clear
End If
End Function