Can't switch the postgres user..

Thank you Roger and Jason.
I downloaded the cygipc-1-14.1 and installed on my XP
Everything goes fine until I do the step:
8. Initialize PostgreSQL (*when running under the "postgres" account*):
mgonzalez(at)guaj215desa ~
$ initdb -D /usr/share/postgresql/data
The files belonging to this database system will be owned by user "mgonzalez
.
This user must also own the server process.
The database cluster will be initialized with locale C.
Fixing permissions on existing directory /usr/share/postgresql/data... ok
creating directory /usr/share/postgresql/data/base... mkdir: cannot create
direc
tory `/usr/share/postgresql/data/base': Permission denied
initdb failed.
mgonzalez(at)guaj215desa ~
$ su postgres
Password:
su: incorrect password
As you see, I tried to change the current user to postgres. I left the
password blank by pressin ENTER twice.
What I am doing wrong (again).
Thank you in advanced for your patience.

MAGO