Well, if you are completely new to all this stuff, then a little more information is in order .
The tools I was referring to (ssh wget) are primarily *nix command line tools that are used from the *nix shell prompt. SInce you indicated you are familiar with Mac, I'm not sure hhow comfortable you will be using the command line, but here is an overview of what is involved:
1) Obtain, and install on your mac, a "secure shell" (ssh) client program (googleing or inquiring in a mac forum should point you in the right direction).
2) Using that ssh client, log onto your account at dreamhost.
3)If you want to run Jommla/Mambo from any location other than your "base" dir (reachable at http://yourdomain.com), create a directory to hold your Joomla installation using the mkdir command as follows
** Note, DO not type the "" above or in the following instructions. This is used to indicate "return" or "enter"
3) If you created a new directory under step 2 above, move into the directory into which you are going to install Joomla
4) Still logged on using your ssh client software, use the DH "wget" command to obtain the latest (as of today!) Joomla distribution tarball as follows:
This will retrieve the latest distribution from joomla.org and will leave a single file in your directory called "frs1304?dl=1"
note for future wget use:
Typically, you will receive a file named in some fashion after the distribution you are retreiving. In this case, because of the link Joomla.org provides, the file name you receive is not related to the distribution name. Generally, to retrieve a file with wget, simply type "wget http://url.to_the_file" and hit carriage return, and the file will be retreived with a meaningful name.
5) Now it is time to "untar" the tarball file you just received by typing:
tar -xzvf frs1304?dl=1
You will now see a long scrolling list of files being extracted from the tarball and directories being built and populated with those files. This is your Joomla installation taking place and, because you did it from the command line logged in as your user, all the files will have proper permissions set (assuming you are running PHP as CGI, which is the default Dreamhost setting - If you are not running PHP as CGI, a different approach is indicated.)
6) Gather the information about your database in which WordPress was installed (Host, database name, user name, password) and use a modern browser to open the directory into which you unpacked the tarball. If you unpacked it into your "base" directory", go to http://yourdomain.com, or if you built a sub-directory for it, go there, ex. http://yourdomain.com/directory
7) Follow the prompts from the Joomla installation screens, iinputing the database information you previously gathered, and you are good to go!