1. Download zip file and place in root folder on your SD card (not in folders)
2. Go into recovery mode.
Power off your phone, then hold home + volume up + power button.
3. Select "install zip from SD card".
4. Select the zip you downloaded.
For ROMs, there is a GT-S5570I Android Development sub-forum. Download and install your ROMs from there.
Sent from my CM7 GT-S5570I