The easy way into Coerthas, which will put you near Miller's Glade in fact, is west from Camp Emerald Moss. If you're not familiar with that, go to Gridania, exit the city, and just follow the road west for about 15 minutes. You can speed this up if you rent a chocobo in Grid.

EDIT: Oh! You found it on the map while I was typing. I hope my tip helps anyway!