domain mapping

can't seem to figure out how this works.
1. using subfolder approach for network
2. have a cname 'beta.replens.dk pointing to wordpressmu-22442-49910-130310.cloudwaysapps.com

pinging the domain i get replies from wordpressmu-22442-49910-130310.cloudwaysapps.com
but trying to map the domain on child site it fails.

Thanks,
Dallas