activity_onDisplayStreamActivityNew doesn't have an activity object yet. You need to access the asset from the $stream, which is in var3 as shown below.
$_PLUGINS->trigger( 'activity_onDisplayStreamActivityNew', array( &$buttons, $viewer, $stream, $output ) );
Note you can find a list of triggers and their variables in the below tutorial.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers
Stream objects don't have a asset property though. You need to call the "asset" function of a stream to get its formatted asset. This can be done using the below format function.
[cb:parse function="asset" class="var3" /]
Be sure to toggle "Format Function" below Custom Value or this won't work.