The amp file is a JSON file at v10.1 and 10.2. The official way to do extensions now requires you to create an MPC assembly and then add your custom task etc to the project through MPC - documented here.However, you can still add the extension or task to the amp file directly with the correct JSON. Example below:
"tasks": [{
"assemblyQualifiedName": "ESRI.ArcGIS.Mobile.Client.Tasks.ViewMap.ViewMapTask, ESRI.ArcGIS.Mobile.Client",
"name": "View Map",
"description": "Browse the map display"
}]
"extensions": [{
"assemblyQualifiedName": "STW.Mobile.Tasks.CustomSearch.SearchTask, SearchTask",
"name": "Search Tool",
"description": "Search Tool."
}]
Note that the amp file is not formatted neatly like this, so you will probably need Notepad++ (this has a plugin for JSON formatting) or something similar in order to make reading the amp file easier. Hope that helps. John