Dreamhost paths are expressed just like any other *nix system.
The way you write the path depends upon where the file is in your directory structure. Assuming your php and .htacces files are accessible to the web your path will look something like this:
/home/yourusername/yourdomainname.tld/directorycontainingthefile/thefile.php (or .htaccess).
yourusername = your machine user name
yourdomainname.tld = your domain name plus the "Top Level Domain (.com, .net, .org) or "whatever directory you told the system to host your domain from when you added the domain in the panel.
directorycontainingthefile = the directory where the file you are tryhing to write the path to is stored (this may not be needed at all if you are storing the file in the "yourdomainname.tld" directory).
thefile.php - the actual full name of the file