Does it cost to join this program?

No. there is no cost to join the program.

Can I stop working with the selected agent and choose another agent?

Yes. You can change agent as often as you want. Just login to your account and request to work with a new agent.

How do I receive credit from the Real Estate Agent?

The real estate agent will credit 25% of his commission to you at closing. The credit will be applied toward closing costs and any loan points (if any).