In English->Portuguese dictionaries/textbooks the Portuguese article is usually an indication of gender and not literally translated. You never see English entries preceded by "the," but you nearly always see Portuguese entries accompanied by the article. It seems translations from Portuguese to English with or without the article should be accepted or at least "almost correct."
I'm with Gaspard's mom - I believe there really is a slight difference in the length of "às" and "as" when spoken in real life. Working with "cut and paste" phrases from a computer may complicate things a bit, but I guess if one is detached from getting all the little red hearts all of the time the learning is still there.
I couldn't tell if the excercise was written or not when I first posted here. I agree - both should be accepted in aural exercises without context. As mentioned by legatrox, there may be a phonetic hint as to which one it is. My mom says that the a in "às" is slightly longer then the one in "as", but I really can't say myself. In reality, they are almost always used in context so this may be moot point.
I agree, both should be correct. I've heard some of the other languages in this program, and even when you should be able to detect a minor difference, the reader is usually not clear enough. Obviously, this is not a major point. But something to keep in mind for the further development of this program.
You don't need a Portugeuse keyboard, see http://french.about.com/od/writing/ss/typeaccents.htm for example.