You should have received an email with your ftp account name and password.
Lets say for example your domain name is example.com
Then the ftp host would be ftp.example.com
Your host directory would be example.com/
Now type in user name and password, then click the test button to verify that it works.
In CS3 you can access the ftp info by selecting site from the menu then manage sites. From here you can set a new account or edit an old one. Since you have not use dreamweaver to access this ftp account you have to set up a new account, this account is for dreamweaver use only and has nothing to do with creating a dreamhost account two seperate things, just in case you were wondering.
I hope this helps,