Right-click (or Control-click) on the Memoir '44 Online app to open the contextual menu and select "Show Package Contents". This opens a new window that contains the app's inside content (an app is actually a folder on the Mac).
Drill down to Contents > Resources > app > lib > macosx
Replace the libjogl_awt.jnilib file by the one you downloaded.
You can now close the window and start Memoir '44 Online.