If you are registered, you get access to the members only section, can participate in the buy & sell second hand forum and last but not least you can reserve your preferred username before someone else takes it.
If it's compatible with Pentax or if it looks like the one in the picture, it's not compatible with Sigma. Pentax has the contact pins under the camera body and sigma has them inside the battery compartment. But, there are indeed some picture in a slide on the left of the page with some picture that look promising but perhaps it will only work with the trigger cable.
You could give it a try and if it doesn't work, I'm sure you'll find someone with a Pentax to sell it to for 36 euros.