The stables are on the road to Levochovo. They are open and working.
If you are coming from Pamporovo, take the road to Smolian. After about 3 km you will pass a big complex on your left called Forest Nooks 3. Keep going for another 800 meters and you will see a turning on your left, with signnposts for 'Levochovo' and 'Pamporovo village'.
Follow this road for about another 4 km (very windy), towards Levochovo. You will pass through the complex 'Pamporovo Village' and some old buildings. After this you will see a sign for the riding stables, where you have to turn right down a dirt track. The stables are about 200 meters at the end of this track. If you arrive at Levochovo village it means you have gone too far (stables are about 1 km before the village).
I suggest stopping off at the 'Pamporovo Village' complex on the way there or back. They have a great cheap restaurant and serve typical Rhodope food. They also have a swimming pool and hot tub which you can use if you fancy a dip !