Use the MouseWheel event of the form:

Private Sub Form_MouseWheel(ByVal Page As Boolean, ByVal Count As Long)
On Error GoTo Err_Handler
   'Purpose:   In Access 2007, make the MouseWheel scroll in Form View.
   '           Many people found this behavior annoying, so MS removed it.
   '           This code lets Access 2007 behave like older versions.

   'Run this only in Access 2007 and later, and only in Form view.
   If (Val(SysCmd(acSysCmdAccessVer)) >= 12) And (Me.CurrentView = 1) Then
       'Save any edits before moving record.
       RunCommand acCmdSaveRecord
       'Move back a record if Count is negative, otherwise forward.
       RunCommand IIf(Count < 0&, acCmdRecordsGoToPrevious,
acCmdRecordsGoToNext)
   End If

Exit_Handler:
   Exit Sub

Err_Handler:
   If Err.Number <> 2046& Then     'Can't move to that record
       MsgBox "Error " & Err.Number & ": " & Err.Description
   End If
   Resume Exit_Handler
End Sub