That's what you want..unsalted butter. Don't add any salt. Well..at least this is what I've always known..and when I have used salted butter the bc has been really yucky tasting. Though I also am not familiar with the Butter cream dream recipe so my opinion may work for you. Does it call for salted butter?
Probably just a couple of pinches. I personally use ALL salted butter in my recipe, so to each his own!
if im making a full batch, i add two "pinches", LOL. i know thats not much help, but maybe try adding some and then tasting it, then add some more if needed. i always add salt cause its wayyy too sweet without it.
I agree add apinch of salt , I just made this icing about 3 days ago and only had unsalted butter, I added salt. tasted great !! a little salt makes everything better 
Here is a good explaination! http://www.ochef.com/553.htm
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