RMA = Return Merchandise Authorization.
Basically, allowing you to return the product to the retailer. In your case, the retailer is not going to provide you with the authorization. Honestly, I don't know if there is any legal grounds for you to stand on here. If you purchased this through an Amazon reseller program, I'd check with Amazon to see if they can offer any guidance.
Otherwise, you're stuck with a warranty claim with the product manufacturer. Register the motherboard and look up the warranty information. Contact the manufacturer and find out what all you will need to send to them in order to get them to accept your claim and repair or replace your motherboard.
Unfortunately, this is often a length process and can take weeks...