Passé Navigo has been super helpful to just have the one card to ride metro, RER trains to/from airport & Versailles, plus worked on Versailles Phebus shuttle bus!
It was though a challenge to obtain as Gare du Nord Stn. did not have service agent for many hours to process purchase. We walked to Gare d'Est to find service agent who could process purchase with photos we took at station photo machines (there are a number of them in each station) - FYI, don't smile for photos - machine rejects as "non-compliant" - mug shot pose works! If you know you will want this pass, you could bring your own photo from U.S. and skip the photo booth step - make sure it's 1" x 1 1/2" to fit on pass. So definitely go for this pass as it's so convenient, but lower your expectations for service to set up as you must buy from agent not from machine - good luck!
Sorry you had trouble at these train stations, but you can get the "kit" (5€) at a info/ticket booth in a Metro station, however, not all have manned booths. I believe you can also buy it from a vending machine, as well. Putting the kit together is a DIY project - you do not need anyone else to assemble it for you. Loading the pass can be done either at a machine or at a manned booth at a Metro station.
While information windows in the metro may require you to purchase the thing on the machine and then once you receive it, charge it there, you pick up the actual kit from the clerk in the information window who should have supplies. I am having trouble imagining that if Gare du Nord was open and running trains that the RER office there was not staffed; that is really quite unheard of. You provide the receipt to this clerk and they give you the kit which you charge on the machine. Or they can sell it to you; you put it together.
A related tip: Before our trip, we color xeroxed our drivers license and cut out the photos to size. We then bought our Navigo Card kits at the CDG airport rail station so we could take the RER into Paris. The photos worked fine.
Any head shot aprox 30 by 25 mm will work fine -- no special paper, not even color required. I always just use snapshots and crop the head and print on the computer. Xeroxing works fine too and you can adjust size on the copier.
Cannot imagine an agent's booth not being staffed? Then you never lived in Paris! LOL
They take their breaks very seriously.
I have spent months in Paris and have not yet encountered a manned booth at an RER station without personnel during the times the trains are running. I know car rentals close shop for long lunches, so I can imagine RER stations might. But I have never seen it.