This is a realtively new museum (September 2011, I believe) and it receives very little publicity. I could find no literature on it and no clear indication of where it could be found. It is a few km out of Sofia and it can be accessed by the Metro and a short walk. My taxi driver had no idea where it was and other people I have spoken to since gave up trying to get there when several taxi drivers said they had no idea where to go. It is actually next to the traffic police (KAT) building (marked on most local maps in the SE of the city).
Even on arrival it is not initially welcoming. Access to the compound is via a secure gate. If there were any signs they were only in cyrillic (a problem all over Sofia). Once inside there is a modest 6 Lev charge.
The place is interesting if you have an interest in this sort of thing. There are 70+ sculptures in the open air and 60+ paintings and smaller sculptures inside an attractive purpose-built building. There is a small shop selling a few souvenirs.
I enjoyed it and it was worth the hassles I experienced in getting there. It could be so much better but with time perhaps it will be.