How to Get to Arx Mentis
In order to reach Arx Mentis you'll need to first complete all of the TDS progression up to and including Combine Dredge. With that out of the way, you'll be flagged to zone into Arx Mentis. Once you're ready to travel to the zone, you'll want to first travel to Combine Dredge which you can reach through Brother Island.
Once you're in Combine Dredge you'll want to travel directly north until you come across the small glowing pillar (pictured below) which you'll want to click on to zone into Arx Mentis.