The amazon services can be complex to use: for one they rely on a complex method to authenticate all the requests. When using the REST functionality of Mendix 6.6 you need to calculate an AWS signature, most likely using a java action, and specify that as a header value.
However, it's probably better to look at another new piece of Mendix 6.6: the Connector kit. Amazon has java client libraries for most of their services. You can use these in some java actions, and write your own connector. The result will be a number of reusable microflow actions for interacting with Amazon.
We will publish a blogpost soon that will describe how to do this. In the meantime you can also browse the sourcecode of some example connectors on github: database connector, Twitter connector
Send me an email if you need more info.