[davidamis]

Thanks for the help, that solved it. Select certainly is picky. I rewrote the sample with some additional explanation, and added selecting an action. If you only want to select a project or folder, leave out the 'tell content' block.

NOTE: code assumes there is a task in a project that is directly in the Library (not in a folder).

tell application "OmniFocus"
	tell default document
		-- example: get a reference to an action (task in Applescript) and its parent project
		set project1 to first project whose status is active
		set task1 to task 1 of project1 whose completed is false
		
		tell front document window
			-- The parent project of an action (task) must be selected before you can select the action.
			tell sidebar -- to select a project or folder, you must tell the sidebar
				select {project1} --must be a list
			end tell
			tell content -- to select an action, you must tell the content
				select {task1} --must be a list
				--The focus may be on the sidebar or the content, select doesn't change it.
				--The following should change focus to the content area:
				set selected to true -- bug in OmniFocus: doesn't work
			end tell
		end tell
	end tell
end tell

The bug that you can't get or set the selected property of the content or sidebar of a window is also shown.

I wrote OFSelectAny() to make it easier to select:
http://forums.omnigroup.com/showthread.php?t=11886