So the web site has been working for a year and was setup by a different user. Yes, you'll have to login as that user.
You haven't provided enough information here to be of any further help.
As I said before, you need to access the web panel and under Domains -> Web write down the username and the directory the files are located in, and you must login to FTP as that user and put the files in that directory.
Did you just transfer the domain to DreamHost, or from another DreamHost account? You might also want to check to make sure the DNS is resolving to the right machine, etc.
If you need furhter assistance, contact Support or provide more details. If you are not ocmfortable posting details, Send a Private message instead ('Send Private' on forum menu.)
Perl / MySQL / HTML+CSS