How to add an onclick event to a joint.js element?

The JointJS shapes are models, so you're right that click handlers won't work on them. The JointJS paper triggers events that might be useful to you:

paper.on('cell:pointerdown', 
    function(cellView, evt, x, y) { 
        alert('cell view ' + cellView.model.id + ' was clicked'); 
    }
);

other events are: cell:pointerup, cell:pointerdblclick, cell:pointermove.

The complete list can be found here: http://jointjs.com/api#joint.dia.Paper:events.

EDIT:

Starting from JointJS v0.9, there is also a cell:pointerclick event.

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