It would be very helpful if you would have given me a start point of where you would be coming from. There are different zones into Pashhow Marshlands and it would be helpful to know if you were coming from Windy, Basty, Sandy or Jeuno.
Since you didn't give me a starting point, I will give you two options and pick the starting point myself. Route one will be from Jeuno, route 2 will be from Valkrum Dunes.
Starting point Lower Jeuno
You can mount a Chocobo at the stable in Lower Jeuno, you will zone out of Lower Jeuno at F-12 into Rolanberry Fields. Make your way west through Rolanberry Fields until you get to H-12. That is the exit/zone that will take you directly into Pashhow Marshlands, at the north most point of the map.
Starting point Valkrum Dunes
In the Dunes make your way to M-11 on the map. You will pass thru the bat cave and out into the area where there are only lizards and goblins. Zone thru the area at M-11, you will know you are at the correct zone if there is a sentry just before you cross into the zone. What Nation's sentry that will be standing there depends on who is in control of the Dunes. If the Beastmen are in control their standard will be there but their sentry/guard will not.
You will appear in Konschtat Highlands. Make your way to K-3 on the Konschtat Highlands map. If you check your map as you are traveling across this map, you will run into the Teleport-Dem crystal at I-6. Make sure to "touch" the Crystal so that you can teleport to it in the future.
PS. If you used your region and Conquest map on your menu, all the info I gave you above is very easy to figure out by exploring those two maps and seeing how each area connects to each other