I just troubled through this for about two hours, due to the mismatched terminology, and the Dreamhost wiki, which is linked to in other forum posts, is out of date.
Live Chat meister Brian A helped me figure this out.
Dreamweaver CS5, aka Dreamweaver 11, wants the following information to set up an ftp connection to a new Site:
Server Name (can be anything-is your own reference)
Connect Using: set to “ftp”
FTP Address: is your domain name only, without the "www." prefix or "/" suffix
Port: can be left at 21
Username: find in your account email from DreamHost or once logged in to your Dreamhost account, it’s listed on the row of the domain, in the “Web Hosting” column
Password: should be in your account email
Root Directory: typically the same at "FTP Address" but with a "/" suffix
(“custom domains” might also need the "www." prefix)
Web URL: the full thing from http to the "/" suffix
Dreamweaver offers “More Options”
"Use Passive FTP" should have a check in the box
The four other options, I left unchecked and was able to connect. (I was concerned that I had set up an IPv6 for my site, and so would need to turn on Dreamweaver’s “Use IPV6 Transfer Mode” option, but I left it unchecked and was able to connect).
So as always, your mileage may vary but this is what worked for me and I wanted to share this with other Dreamweaver CS5/Dreamweaver 11 users.