Postgres requires a user because in Unix all services often have their own user for security reasons. It limits the possibility to take over the whole system if it got breached.
Now OSX Lion comes shipped standard with PgSQL, so there is no reason to install it.
The binary can be found at:
/usr/bin/psql or type in the cmd
psql -d postgres -U postgres
If that fails, and you have updated your system from Snow Leopard to Lion:
It's a PATH issue. Mac OSX Lion includes Postgresql in the system now. If you do a which psql you'll likely see usr/bin/psql instead of usr/local/bin/psql which is HomeBrew's correct one. If you run brew doctor you should get a message stating that you need to add usr/local/bin to the head of your PATH env variable.
Editing your .bash_profile or .profile, or whichever shell you're using and adding: export PATH=/usr/local/bin:$PATH
as the first export for the PATH then either quit you shell session or source your file with source ~/.bash_profile and it should now be OK again.
You can also edit /etc/paths and make sure /usr/local/bin is on the top.
If for some other reason you still want to add a user:
# dscl / -create /Users/portingunix
# dscl / -create /Users/portingunix UserShell /bin/bash
# dscl / -create /Users/portingunix RealName "Porting Unix
# dscl / -create /Users/portingunix UniqueID 503
# dscl / -create /Users/portingunix PrimaryGroupID 1000
# dscl / -create /Users/portingunix NFSHomeDirectory /Local/Users/portingunix
# passwd portingunix