On Galaxy S with Gingerbread, you have Recovery 3e which requires the ZIP files to be digitally signed otherwise it wouldn't allow you to install unsigned ZIP.
You should first get rid of 3e recovery. I have personally installed ROM Manager from market and using it , I have CWM recovery which allows you to toggle signature verification so that you can install unsigned ZIP.
After you install CWM, click it and select Reboot into CWM recovery. From there you can do what you want.
I hope this helps