To do so:
Step 1: Download and install Datafield mobile application on your device. Here are the links to get the app:
In the iOS AppStore: http://itunes.apple.com/fr/app/datafield/id470573506?mt=8
In Android GooglePlay store: https://market.android.com/details?id=datafield.src
Step 2: Open Datafield mobile application and login to your account. You and your mobile respondents can simply input the email and password received while their account been created in the #manage mobile team panel control. More details on managing field respondents here.
Alternatively, field respondents can also simply input your organisation TAG into the first text field and hit the GO button. In this case, mobile respondents will be logged as public users.
Check out this video tutorial: