"But I have been marked as incorrect when using it" - Could it be possible that you were still using the old spelling? There was a major reform in German spelling that started in 1996. The "ß" hasn't disappeared entirely, but many words are now written with "ss" instead (e.g. "Ich weiß, dass du kommst" - "dass" used to be spelled as "daß", which is now wrong). See: http://en.wikipedia.org/wiki/German_orthography_reform_of_1996#Sounds_and_letters

I'm not sure I understand what you mean. From what I've seen, the letter "ß" appears in the "character area" of Duolingo's German course whenever you have to write something in German. There is no official upper-case version of this letter as it never occurs at the beginning of a word. In recent years, some typographical suggestions have been made to overcome the problem of writing "ß" when using all-caps. But they don't really have an official standing yet. When using all-caps, you normally have to replace the "ß" with "SS" according to present spelling rules. I'm typing this on a German keyboard and even I don't have a capitalized version of "ß". :) For more information, see: http://en.wikipedia.org/wiki/Capital_%E1%BA%9E

Thank you. I see where it is now; for some reason I missed it before. But I have been marked as incorrect when using it, so I still have a concern. My German is quite good, just rusty, so I don't want to confuse myself with an inconsistency between the way I learnt and duoLingo.