Margarte is a Nord merchant in Leyawiin. She can be found either in the Great Chapel of Zenithar in Leyawiin, in her house directly across the street from the main entrance to the temple, or just outside the city collecting alchemical ingredients.
Modryn Oreyn sends the Hero to Leyawiin to investigate what has happened to three other Guild members. Apparently, they have been drawing a lot of negative attention towards the Fighters Guild and causing trouble at the Five Claws Lodge.
She is available for Journeyman Mercantile training whenever she is not sleeping (12:00 AM to 6:00 AM), eating breakfast (6:00 AM to 8:00 AM), collecting ingredients (12:00 PM to 6:00 PM), doing Sunday worship (Sundays from 8:00 AM to 10:00 AM), or eating dinner (8:00PM to 10:00 PM).
This means shes available for training most days from 8:00 AM to 12:00 PM and again from 6:00 PM to 8:00 PM, and from 10:00 PM to 12:00 PM. She will most likely be at the chapel during those hours.