Here's a couple of links, I think Turkmen was the original site, it's also a lot closer to the beach!
http://www.olymposturkmentreehouses.com/olympos/index.htmhttp://www.olympostreehouses.com/BTW, are you driving? the reason I ask is that whilst the bus / dolmus will get you to the road that leads to the small village, there wasn't any transport down this road, we ended up hitching. I guess though that if you make a reservation then the treehouse may pick you up from the top; worth calling ahead.
There are also some pansiyons along from the beach that are worth considering, as they were slightly less touristy and were a bit cheaper but we were there in 97, so things will probably have changed.