You need to use a modified version of Standard Firmata. I have a copy in s2a_fmhttps://github.com/MrYsLab/s2a_fm/tree/master/Snap!Files/Snap!Mobile/StandardFirmataYun/StandardFirmataYun
I am using this version with the PyMata https://github.com/MrYsLab/PyMataclient that runs on linino.
I have an example of controlling a DC motor over wired and WiFi in this Instructable http://www.instructables.com/id/The-SnapMobile-Start-Your-Physical-Computing-Engin/