Find the mound of Soft Mud and click on it. Maarinette will appear not far from there, and you'll be able to talk to her.
This mission appears to be bugged, as sometimes Marinette doesn't appear. This can be corrected by grouping with other players, and letting them click on the Soft Mud.
Gyasi Sel wants you to make a sacrifice to Marinette of the Dry Arms.
- Bury a Sacrifice to Marinette-Bwa-Chech
- Speak with Marinette