I know you can trade ZET at cryptsy.com
However, that Exchange is very unreliable at the moment due to high load so everything takes ages. But if it works:
1. Register
2. Click on Balance
3. Click the button on the right side of the coins you want to deposit
4. Select "deposit"
5. Create a new deposit adress (NEVER press that button if you already did and while you are waiting for an incoming transaction!!)
6. Send coins to that address
7. Be patient
8. Be patient
9. Be patient
10. Buy Bitcoin if you deposited another coin
11. Buy ZET
12. Good luck!
If you don't own any coins you need to buy some for "real" money on an Exchange like mtgox.com or btc-e.com or ...
Then add 5x "Be patient" :/