Usability of customer registrations

I am doing more testing on the marketpress plugin and I am confused about the workflow for a customer perspective, it seems to not be working.

1. Customer adds product on a domain mapped store.

2. Customer goes to shopping cart and clicks

3. Customer clicks "login and check out"

4. Customer is then brought to the "main domain" branded login page. (this is perhaps confusing to a customer wishing to buy a product from the mapped domain website)

5. If the customer then clicks "register new account" it brings the customer to the top level domain to sign up for a new site.

FYI I have sploging plugin installed, prosites, domain mapping, and marketpress all installed.

Am I setting this up wrong or is there perhaps some review that needs to be done on this process from a customer standpoint.

Thanks guys.