http://forums.asp.net/t/1127834.aspx

you can use the following javascript function to set the active tab: 

function SetActiveTab(tabControl, tabNumber)
{
  var ctrl = $find(tabControl);
  ctrl.set_activeTab(ctrl.get_tabs()[tabNumber]);
}

tabControl: ID from the TabContainer Controls
tabNumber: Number of the new Tab (starting at 0)

Or

$find('<%=TabContainer1.ClientID%>').get_activeTabIndex();

and

$find('<%=TabContainer1.ClientID%>').set_activeTabIndex(2);