Sorry for my bad English, it is not my native language.
I am dissapointed, that there are no information in WordPress plugins about 777 (I think, you understand, about what it). It is not problem of WPMU Premium only: it is everywhere.
I think, all WPMU Premium plugins must have in their readme files information about doing 777 for directories.
P. S. I was not understanding, why plugin Avatars did not wanted to work. The problem was in that, because I did not know, that I must do 777 for directory with avatars (there are were no words about it in readme file).