The first Wordpress admin login is where you create the first admin user. Follow the link in the email and you are asked to create the first admin user.
Noting your other thread, it looks like you were waiting for DNS propagation when you did your one click install.
If someone else got there first, and you don't know who, consider your wordpress installation compromised and start over. (Use a different "user" for the domain, and let the one click installer create a new database.) (if you need more detailed instructions just ask)
To avoid this issue it would be nice if dreamhost would warn somewhere to wait for the "coming soon" page to appear on your domain before running the one click installer and let advanced users click through it because there are cases where you might want to proceed, but most beginners get hung up because an email arrives and the link doesn't work quite yet.