Well if your friend gave you access to his linode manager account or a sub account then you can add DNS records there. If not then it's probably easier using your registrars name servers and pointing A records to your friend's linode's ip. Either way name servers are set with your domain registrar
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Paid supportHow to ask for help
1. Give details of your problem
2. Post any errors
3. Post relevant logs.
4. Don't hide details i.e. your domain, it just makes things harder
5. Be polite or you'll be eaten by a grue