It seems like sx150 has been not ported yet. This is a work in progress as I can see. For my sx130 it took around half an year (as far as I remember) before CHDK run on this camera.
You can just wait for the port. There's no varranty that this be ever done. It depends on people (and camera itself).
You can also help porting. Just ask on the thread if there's something that you could do. Sometimes even if you don't know anything about programming you can still become a tester

Ask more on this thread:
http://chdk.setepontos.com/index.php?topic=6953.105