It's easy
First, you have to set the flash on camera (only for the first time), turn it (and camera) on, activate wireless function by going Sel->Remote->Slave. Then, go to Para->Channel, select whatever you want (default 1 is ok unless there are lots of other Pentaxians around doing wireless-flash shooting), this will set the same channel on flash and camera
Second (anytime when you want to use wireless flash)
1. Take the flash, set it to Sel->Remote->Slave
2. Take the camera, go to flash menu (arrow down), select wireless flash (the last symbol in sequence, with a "W" letter over the lightning). You have 2 options on K-3 (earlier models required digging in cFun to ajust the flavour). Wireless master means that in-built flash not only controls the slaves, but also lights the scene. If you choose Control, then in-bult flash tries not to lighten your subject. Pop the in-built flash (this is critical step!). Those having K-3II will have to mount Master-capable flash on top of camera to go wireless.
3. Take a test shot. Most common problem is that light sensor on Metz doesn't see the control flash impulses. That sensor is located on top of flash enclosure (not head, but "body"), on the left side. I usually rise the head upwards and turn it 90deg to clear sky for sensor, then it works fine even without direct visibility (in a moderate sized room).
4. You can control flash options via Para menu, including EV comp, zoom, etc, and switch on/off secondary reflector as usual (via Sel menu)
that is the sensor (little clear cap):
make sure it's not blocked by flash head or anything else