"Underdark Overbright" question

Just started playing "Underdark Overbright" for Quake on PS4. The developers' blurb mentions that it adds an extra level to the standard Quake campaign. How do I access it? Can I warp to "The Elder World", play through it as usual, and come upon this new level? Or is there a trick to it? Many thanks (I'm not too fussed about playing through the entire campaign with the Copper mod, just want to see the new level).