Open Mail Preferences and select the "Rules" pane. Because using a mouse is so 2012.

gistfile1.applescript

tell application "System Events"
  tell process "Mail"
		tell menu bar 1
			tell menu "Mail"
				click menu item "Preferences…"
			end tell
		end tell
	end tell
end tell
tell application "System Events"
	tell process "Mail"
		tell window 1
			tell tool bar 1
				tell button "Rules"
					perform action "AXPress"
				end tell
			end tell
		end tell
	end tell
end tell

Hey thanks. Really I'd like it to edit a specific rule, but for now I'm adding this to a Mail Act-On rule so a keystroke will take me straight to the rules menu. A start...