Looks like Message Media does not have a RESTful interface. To use Message Media's SOAP interface, you will have to setup a webhook subscriber service which receives the webhook POST from Marketo and forwards to Message Media using their SOAP interface.
The setup would be similar to what is described in https://community.marketo.com/MarketoArticle?id=kA050000000L8dH&src= except that the subscriber service will invoke the Message Media SOAP API to send the SMS.
Since this thread is a bit dated, I'm not sure if you're still looking for an SMS solution that's integrated with Marketo via webhooks. If you are, you may be interested in this whitepaper that we put together: http://ww2.ringlead.com/CoolThingsYouCanDowithWebhooks_Download_b.html
This document gives an in depth solution to integrating Marketo and Twilio.
I hope this helps!