LEO does, however, have some usage markings, and people who use it are supposed to know that they need to consult a print dictionary for more detailed explanations. My hunch is that this word probably appears in many print dictionaries (which it's Nobby's job to cite), and if so, I see no reason for LEO to exclude it.
'Dialect,' well, maybe. The US (like, as far as I know, Australia) doesn't really have as many distinct dialects as the UK and Europe; in most cases we just have regional accents, not sharply different vocabulary and grammar. This one word might qualify as dialect, but I suspect it's more like sociolect. To my ears, youse is distinctly working-class -- probably a good deal more so than y'all.
I agree that LEO can't mark that, but Doris could at least link to this thread, and if there are other regional factors affecting usage, people could comment on them. (I'm still waiting to hear about Ireland, which I suspect was the link to the US & Australia.)
Again, though, neither 'youse' nor 'y'all' is slang; that's simply wrong IMO. Slang is basically metaphor, symbolic language, using a word to mean something else -- for example, using a word like 'dough' or 'Knete' to mean money. So I think the correct marking is probably just [ugs.], even though there are different degrees of colloquialness and this is a less standard one.