From the Outlook object model, Application.ActiveInspector.CurrentItem, will return the currently open item. Cast it to an AppointmentItem, and you will have access to all the AppointmentItem properties. Here's a VB.NET snippet:
Code Snippet
Dim olApp As Outlook.Application
Dim insp As Outlook.Inspector
Dim appt As Outlook.AppointmentItem
olApp = Globals.ThisAddIn.Application
insp = olApp.ActiveInspector
If insp IsNot Nothing Then
appt = TryCast(insp.CurrentItem, Outlook.AppointmentItem)
If appt IsNot Nothing Then
MessageBox.Show(appt.Start.ToString)
End If
End If
