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You have to reference the master 1 or 2 with its document.
So try this.

tell application "OmniGraffle Professional 4"
set fw to front window
set fd to fw's document
-- set shown canvas master
set fw's canvas's master to fd's master 1
-- set a specific Canvas of document to a master
set fd's canvas 1's master to fd's master 1
end tell
Best Regards