The CRM UI also calls the Book message when creating a Service Activity, so you could try registering your plugin on the Book event.
However, if all you need to do is ensure the location field has a value, you could just make it a mandatory field. Or, if the logic is a bit more complicated, you could use client script in the OnSave event
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk