That Library folder is at the root level of your boot drive.
From the Finder, choose your drive from the sidebar of the finder window.
If you are not familiar yet with using the Finder, click on the Mac (smiley) face at the left end of the dock. You'll get a Finder window.
Left side is the sidebar, so click on your drive, under Devices.
You'll see a list of the directory on your hard drive. Double-click Library, and then double-click the Application Support.
And, you'll see the ProApps folder.
There's several other methods to get to that folder, so experiment a little.
One hint for the example that you asked about: Ignore the word "Volume", as the path is really /Library/Application Support/ProApps