Hi there,
If it's listed under warranty when you check with the online checker you won’t need the receipt to go for RMA. All the information you need is here:
wdc.custhelp.com/app/answers/detail/a_id/1222
Basically, you first need to apply to get an RMA number online, than make sure you follow the packaging instructions and send it to the proper address for your region.
Before you apply for RMA is good to make sure that the drive has failed, as it could be something else that is causing issues.
Hope that helps.