How to change register form on Force login shopping cart

We have modified the registration form of WPMU, and we want to add that to MarketPress shopping cart page.

Can any folks here tell me where we can find the code/files in MarketPress Plugin to modify the "new user register form" block on the right of the page /store/shopping-cart/? The screenshot is below:

http://screencast.com/t/w2etnhLH4

Thank you for your help :slight_smile:

Note:
1. We set MarketPress to "Force Login" in Store settings>Miscellaneous Settings.

2. We use "Buy Now" button.

When people click Buy Now, they will see the screen above asking for login or register. We want to change the register form on the right of the " /store/shopping-cart/' page when the user is not login yet.

Thank you!