If you are living in Yugoslavia to learn more about your family and you say that they are from Kotor then I advise you to go to Kotor and go to the historical archives there. It is possible that your people came not from Kotor itself but anyone of the many villages nearby. They just told others that they came from Kotor because that was the nearest town.
It is also possible that people with your surname live in the Kotor area and can point you towards the village. If one is pinpointed go to the church where the records are kept (for the most part from the years 1825-1945) It may well be that the parish priest wrote in the family page on your family that one of the children went to america. I have seen those entries.
Of course you should learn Serbian and Cyrillic script. The books will be in that and handwritten at that.
You might also try Nakecenovic's book Boka which was written in 1913 and identifies various villages with surnames. Particularly good for rare names. I am afraid with a name like Popovic you will have many leads to follow.
By the way the people in Kotor are great! and the scenery is one of the tops in the world. Still unspoiled.