unable starting / stopping postgresql with pgAdmin III on OS X

I've tried to activate the start / stop option for the postgresql
service in pgAdmin III on my Snow Leopard Installation. The PostgreSQL
Version is 8.4.
I can start the service via terminal with:
geo-mbp:~ gm$ sudo su postgres
bash-3.2$ /Library/PostgreSQL/8.4/bin/pg_ctl -D
/Library/PostgreSQL/8.4/data start
server starting
bash-3.2$ /Library/PostgreSQL/8.4/bin/pg_ctl -D
/Library/PostgreSQL/8.4/data stop -m fast
waiting for server to shut down.... done
server stopped

How can i use these commands in the GUI of pgAdmin III to start and stop
the service? I've tried the following and a few other things with no
success in the Properties pane under service:

sudo su postgres -c /Library/PostgreSQL/8.4/bin/pg_ctl -D
/Library/PostgreSQL/8.4/data

What's wrong with that????

I've switched from Windows to OS X and would apreciate any help.

Thanks in advance

--
Georges Moes