Never fear; For I... Am here!
Anywho, the effect your looking for is that of chain lightning... Am I right?
Okay, theres several problems with adding this effect to the spells...
Chain lightning effect is added via the "Lighting" options in the spell, theres quite a selection to choose from. Ranging from life drain to lightning bolt; Selected it via there...
However. This effect doesnt work on all spells, and thus. You will have to make it via Lighting > Create Lighting from point X to Point Y.