For the folks who DON'T have a hardware MIDI keyboard, it's still recommended that you use one. But in a pinch, the good folks at Sibelius have written up an article on a couple software alternatives.

[url=http://www.sibelius.com/cgi-bin/helpcenter/show.pl?id=403&languageid=1][b]Check out the article here.[/b][/url]