http://developer.appcelerator.com/question/148047/alloy-tabgroupsetactivetab-functionality

app_controllers_index.js

// save a global reference to the tabgroup that can be 
// accessed in any controller
Alloy.Globals.tabgroup = $.index;

$.index.setActiveTab(1));
$.index.open();

app_controllers_tab1.js

function doClick(e) {
    console.log('Label Click From Tab 1');
}

app_controllers_tab2.js

function doClick(e) {
    console.log('Label Click From Tab 2');
    
    // This doesn't work because you are trying to access
    // $.index, which doesn't exist in this controller.
    // If you want to access the tabgroup, use the global
    // reference we saved in the index.js controller
    // $.index.setActiveTab(0); // THIS DOESN'T WORK!!!!
    Alloy.Globals.tabgroup.setActiveTab(0);
}

app_views_index.xml

<Alloy>
    <TabGroup>
        <Require src="tab1" />
        <Require src="tab2" />
    </TabGroup>
</Alloy>

app_views_tab1.xml

<Alloy>
    <Tab id="tab1" title="Tab 1">
        <Window id="winTab1">
            <Label id="t" onClick="doClick">My Label</Label>
        </Window>
    </Tab>
</Alloy>

app_views_tab2.xml

<Alloy>
    <Tab id="tab2" title="Tab 2">
        <Window id="winTab2">
            <Label id="t" onClick="doClick">My Label</Label>
        </Window>
    </Tab>
</Alloy>