Access an S3 bucket created by another user

Question

Here's my scenario: I'm user Marius, and all I want to do is to:

1. access an S3 bucket created by user Admin,
2. upload some images there,
3. access them, via CloudFront, later on.

Here's a summary of my attempts so far:

1. User Admin, using the AWS Management Console (AWS MC), grants List, Update/Delete, View Permissions permissions for his already created S3 bucket to User Marius.
   To grant permissions we used this approach, using for grantee first the users email; then the AWS canonical userid, both of them falling back to the username.

2. User Marius uses CyberDuck to successfully connect to S3, using Access Key ID / Secret Access Key, but the root folder is empty. Accessing S3 through AWS MC doesn't help either:

   To get started using Amazon S3, create a bucket to hold your objects.

The attempt to create a new bucket with the same name -- as per Tom Andersen's comment below -- fails: the bucket already exists.

What do I have to do to access that S3 bucket and modify it's contents?