You only will need to change the nameservers if you purchased your domain name elsewhere.
If you purchased your domain name here with your MMCart and hosting; you don't need to worry about it.
All registrars are slightly different, but most have somewhere that says "Manage Nameservers" or "Manage DNS".
This example uses the popular GoDaddy -
1. Browse to: http://www.godaddy.com
2. Login using your Login Name or Customer Name and Password. If you have forgotten your username/password, use the password retrieval link on this page (Look for the "Forgot your password? Click here." link).
3. Click on: My Account
4. Click on Manage Domain Names and select the domain name you are using
5. Click on the link for Nameservers at the top
6. Enter merchantmoms's name servers as directed in your Welcome Email
7. Click Save