Enums are just list of possible values, you need to use a property or object
that takes the enum as its type.

Going into c:\Program Files\Microsoft Office\Office11\1033\VBAPB10.CHM (the
Object Model Reference) and searching for Zoom brings up the entry for the
Zoom property.

====
Zoom Property
See AlsoApplies ToExampleSpecifics
Returns or sets a PbZoom constant or a value between 10 and 400 indicating
the zoom setting of the specified view. Read/write.
PbZoom can be one of these PbZoom constants.
pbZoomFitSelection Resizes the page view to the size of the current
selection.
pbZoomPageWidth Resizes the page view to the width of the publication.
pbZoomWholePage Resizes the page view to the size of a whole page.

expression.Zoom
expression    Required. An expression that returns one of the objects in the
Applies To list.
Example
The following example sets the zoom for the active publication so that the
entire page fits on the screen.
ActiveDocument.ActiveView.Zoom = pbZoomWholePage
====

Clicking Applies To gives the entry for the View object, which is normally
accessed through the ActiveView property of the Document object.

e.g. ActiveDocument.ActiveView.Zoom = pbZoomPageWidth