This code will zoom display to extent of first layer in TOC(Table Of Content).
Public Sub ZoomToLayer()
Dim pMxDoc As IMxDocument
Dim pMap As IMap
Dim pActiveView As IActiveView
Set pMxDoc = Application.Document
Set pMap = pMxDoc.FocusMap
Set pActiveView = pMap
If pMap.Layer(0) Is Nothing Then Exit Sub
pActiveView.Extent = pMap.Layer(0).AreaOfInterest
pActiveView.Refresh
End Sub
