How can I treat this butt dent? (Photo)

Hi, 
If this dent is natural and not created from a procedure, my best approach would be to use a process called carboxy therapy. The carboxy therapy will be able to soften the connective tissue causing the dent as well as soften the excess fat around the dimple so that the contrast is diminshed and the dent is less apparent. Alternatively, you can try using a combo radiofrequency/tissue manipulation like velashape 3. This treatment will also loosen the connective tissue that is creating the dent as well as soften the contours of the fat around the dent while tightening the skin which will all make the dent less apparent. 
I hope this helps with your concern
All my best
Dr. Jamé

Hi and welcome to our forum!
Several questions...Have you had previous surgery or trauma to the area? I note what appears to be a bruise in the vicinity of the indentation...Is that the result of suction therapy or is the discoloration permanent? 
Suction therapy causes swelling in the area of treatment, camouflaging the indentation. The result is short lived. The fasciablaster is a device which is reported to "loosen fascial adhesions" to treat cellulite. 
If the indentation is of natural origin, lipoinjection into the area may fill out the recess. If the indentation results from previous surgery or trauma, scarring may be responsible for the indentation. This is more difficult to treat, but again lipoinjection will most likely result in improvement of contour. 
You may consider a consultation with a board certified plastic surgeon for examination and recommendations.
Best wishes...