I made some custom buttons that would change the extent. Not sure if this is what you are looking for or not: Here is the code for those buttons:
'When Plus sign is pressed
Sub btnZoomIn_OnClick
'Zoom in
Set extent = Map.Extent
extent.ScaleRectangle 0.5
Map.Extent = extent
End Sub
'When minus sign is pressed
Sub btnZoomOut_OnClick
'Zoom Out
Set extent = Map.Extent
extent.ScaleRectangle 2.0
Map.Extent = extent
End Sub