Title says it all.
I have a domain registered at Dreamhost.
I have setup a fully hosted e-mail address.
When I try to access it at webmail.mydomain.com, I just end up on the mydomain.com homepage.
I can only guess what’s going on here since you didn’t share your domain name… Which DNS server are you using to resolve your domain? If it’s not DreamHost nameserver then most likely there is no knowledge in the global DNS system of the existence of webmail.mydomain.com. Quick fix is to just connect to https://webmail.dreamhost.com.