You could use the DreamHost Panel and under Users|Manage Users click the 'login now'button.
This will take you into the webftp program.
Click the blue folder icon next to your domain name.
When you have arrived at your domain name folder you can click the button at the top labelled 'New Dir' and just follow the instructions to name your folder and finally press the green tickmark to accept.
You can do the same under your own ftp/ssh program by creating a folder under your domain name.
(By creating your folder under the domain folder you will ensure that it's contents can be read by the web server.
Opinions are my own views, not DreamHosts'.
I am NOT a DreamHost employee OK!! :@
You act on my advice at your own risk!