Using Finder, how can I view /System/Library?
Press ⇧⌘G (Shift-Command-G) to bring up Go to Folder and type in the path, then click Go as shown in the image below. Or if using the mouse or trackpad, click: Finder > Go > Go to Folder
You can also change Finder Preferences to show Hard disks on the Desktop under General and or in Devices under Sidebar, thus being able to open the Macintosh HD from the root of the partition without having to use Go to Folder.