In the Knowledgebase article it says: "...To prevent [errors], make sure you can ping the same IP for both the master and addon domain before creating the addon domain..."

I changed the DNS (of the domain I want to add) about 10 hrs ago, and 2 hr ago, ping returned no reply on it.

Now it replies and has a NEW IP but it doesn't match the IP of my Glowhost account master-domain. When you said "ping the same IP" is that what you meant?