Okay, let me try to help with this some.
First question, did you use the one-click installer from Dreamhost's webpanel to install Wordpress for you or not?
That is the simplest way to have it installed on your site.
Click where it says Wordpress and follow the steps to put it where you want it to be. You will want a custom installation, which is the big blue bottom that says Custom Installation on it.
Select your domain in the menu that appears, if you want it on the top level of your site leave the box to the right of your domain empty. Otherwise type the name of the folder you want it installed in on your site.
Regardless of whether you are installing on the top level or in a folder IT MUST BE EMPTY OR THE INSTALL WILL FAIL!
So, if you have a folder like DOMAIN.COM look inside, it should be empty. Or if you want DOMAIN.COM/STUFF that must be empty.
Click install it for me now and it will be.
Once that is done, you will get an email from Dreamhost saying it has been installed and you can then continue with the steps in the Codex AFTER you complete the installation by following the few steps in the email.
After that you should have a Wordpress multisite installation up and running.
Having looked at your domain, you have a WP install on it already.
So, at this point you would open filezilla, login to you server and go to the top level of your site where you will find
right click on that file and select EDIT from the menu choices.
A new window will open showing you the contents of the file.
/* Multisite */
above where it says /* That's all, stop editing! Happy blogging. */
DO NOT put everything on one line. one line for each thing
If you don't see that, then look for require or include at the beginning of a line and put the same thing above that.
Then save the file and continue as the Codex tells you to do.