You may already be doing this, but just in case...
Step 1) Mount the flash on the hotshoe of the camera. Turn the flash on, then turn the camera on.
Step 2) Hit the "Fn" button and go into the flash menu. Select wireless mode (the lightning bolt with the W) You'll notice in top LCD screen on your camera that "CH 1" is displayed. You can change it to 1-4, but the only reason to do so would be if you're shooting an event with another photographer, pTTL does not support individual flash zones.
Press the shutter button half way to activate the meter. You should notice on the back of your flash that "pTTL" is now displayed instead of "TTL".
Step 3) Remove the flash from your camera. Hit "sel" and arrow down until you get to "remote", hit the enter button and then arrow down again until it says "slave". Now position your flash in the room. On the left side of the flash (if you're looking at the back display panel) about two inches above the USB port is a tiny IR sensor, that is the optical slave. However you aim the flash head, make sure that sensor has a clear line of sight to the camera.
Step 4) Pop up your on-camera flash, or mount a second flash on the hotshoe, and you're ready to shoot. You can dial in anywhere from +1EV to -2EV flash compensation from the camera's flash menu. By default the on-board flash will contribute to the exposure, if you don't want it to do so, look for an option called "flash in wireless mode" in your camera's custom menu and set it to "off" (it's #29 on the K20d, but might be different on the K5)
I know it sounds like a lot, but practice a few times and it becomes second nature.
Note that you can also adjust EV compensation on the flash itself and it has a cumulative effect with any you dial in on the camera (so +1 on the flash and +1 on the camera yields +2). This is useful for multi-light setups, say if you want your fill light to be one stop less than your main, you can select -1 on your flash unit providing the fill, set your main unit at 0, and adjust both in parallel from the camera's flash menu.
Let us know how it works, and feel free to "like" this post if it was helpful