It seems this API is only available in the US. From their main page for this API:
Amazon Mobile Associates Availability: The Amazon Mobile Associates API serves U.S. users and works with mobile apps on Kindle Fire, Kindle Fire HD, and Android phones and tablets.
Also, looking at the detailed API Reference, I could only find a marketplace
parameter in the User
object (and even without further digging, being part of the user object it probably won't do what you are looking for).