There's no direct route.
From 24th July, Volareweb start a daily service to Milan Malpensa:
http://buy.volareweb.com
From there, it's an hour and 10 minutes on the bus to Milan Linate, for �10:
http://www.sea-aeroportimilano.it/en/malpensa/ index.phtml?mod=per_mi_scroll&to=come_arrivare
From Milan Linate, easyJet operate 2 flights per day to Palermo:
http://www.easyjet.com/en/book/index.asp
(Alitalia currently operate 3 flights per day on that route but, since they're expected to cease trading any day now, I wouldn't recommend booking with them).
The alternative (which might well be cheaper anyway) is to travel via Athens and Rome. You can purchase a through ticket with Aegean Airways. For a random date I've just checked on their website (14 May), here's how it works out:
Depart Heraklion 1500
Arrive Athens 1550
Depart Athens 1815
Arrive Rome Fiumicino 1925
Depart Rome Fiumicino 2120
Arrive Palermo 2220
The total fare is �216.34 (inc t&c).
Chris