Which door to enter if just attending mass at Notre Dame

According to emilyofnortredame, "There is a separate "Mass access only" queue, which is in front of the central portal, and is marked by a small white sign (please note the sign doesn't get put out until 30 minutes ahead of the Mass start time). Once you enter Notre Dame, you'll go directly to the Nave (the centre) to take a seat for Mass/the service. Entrance still subject to the available capacity inside the cathedral."

Reference https://www.notredamedeparis.fr/prier/messes-et-offices-nd/#:~:text=La%20Cath%C3%A9drale%20Notre%2DDame%20de,au%20portail%20du%20Jugement%20dernier
A dedicated line opens 20 minutes before the service, via the entrance opposite the Last Judgment portal.
The chairs in the nave are reserved for those wishing to attend these services.

We were just there two weeks ago. As the others note, there is a separate line for mass attendees that starts in the courtyard in front of the Cathedral, right next to the line for other visitors. Clearly labeled, you can't miss it. We got there maybe 30-35 minutes before mass and at that time the line was short enough that we able to get good seats. But the line was filling up rapidly behind us.

Last Monday, I just walked up to the front entry for the 5:30 Vespers, Mass, and procession to the close the door as the Cathedral closes after Mass. Arrive before the Vespers if you would like a good seat for the Mass.

when we went some weeks ago, the line for the mass was on the right (as you are looking at the front of the cathedral). A little hard to see with all the crowds waiting for the regular entrance, but just go to the right and keep your eyes out for the sign.