It will be in
C:\SmarterMail\Domains\<domain>\Users\<user>\
If it's their only calendar it will be called, AE1279A3_calendarData.xml. Otherwise you will have to cross reference the UID found in <CalendarView> (in the userConfig.xml) with the ID found in the _calendarData files.
As far as merging goes, you can try to go the bottom of the current xml and see if there were any additions (<Events>'s) and move them into the backup before replacing the original with the backup.
Matt Petty
Software Developer
SmarterTools Inc.
(877) 357-6278
www.smartertools.com