Here's a step-by-step tutorial for using the S3 Media Source that comes with MODX Revo. If your client already has an S3 account, you can skip the first step. You cannot retrieve the secret_key, if your client doesn't have it you must create a new one. Some useful links at the end.
1. Sign up for a free Amazon account.
2. Create your key and secret_key. Download the file, and make sure to put it in a safe place.
3. Create a bucket and put some stuff in it.
4. Check the path "bucketname.s3.amazonaws.com" to make sure you get the .xml file
1. Create a new Media Source
2. Edit the source (don't worry about the "type" column, that's just what kind of field your details use)
3. Put in the URL you checked above.
4. Put the name of the bucket.
5. Put the key.
6. Put the secret key.
7. Save it.
Go to the Files tab, select your new Media Source, and your bucket should open to display your files.
http://aws.amazon.com/documentation/s3/
http://www.cloudberrylab.com/blog/how-to-find-your-aws-access-key-id-and-secret-access-key-and-register-with-cloudberry-s3-explorer/
https://console.aws.amazon.com/iam/home?#security_credential (must log in to AWS account)