File Download Supprot

I would like to update the marketpress plug-in to support Amazon S3 files which do not have their ACL set to read for everyone and use my access keys + the S3.php file (http://undesigned.org.za/2007/10/22/amazon-s3-php-class). Can someone point me in the correct direction so I don't have to dissect the code?

Thanks,

Andrew