please help it Wont let me join private worlds

No matter what I do or try i cannot join my friend's private worlds. All it will say to me is "unable to connect to world" even though their name and world shows up in the friends tab. I am severely annoyed and upset by this because the whole reason i bought this game on my computer was so I could play it with my friends, but i cant even do that. If it doesnt work i at least would want my money back because the stupid game doesnt work when it comes to multiplayer.