First step is add the domains as "fully hosted" on the manage domains page in the panel. (this is different than registering the domain.)
Second step is WAIT. Until you can visit the domain you just added with your browser and see the generic dreamhost coming soon page.
Step three go to Goodies > One Click Installs in the panel and use the one click installer to install wordpress. (hint: you probably want to leave the blank after the / empty -- it can be left empty.)
You will get an email with a link to click to complete the install. (and if you skipped step 2, then that link probably won't work yet)
Added note: the wait for the "generic dreamhost coming soon page" will be anywhere from a few minutes to 24 hours.