Hey Colin,
Sure, post away!
I'd put a note with it letting others know it is not a fully produced demo, just so they don't comment on that part of it.
As an alternative, you can always post individual spots in the Leads and Reads area instead. People usually post dry stuff there since it's for auditions...
As for "How", you can either post a link to it (if it's online somewhere), or you can attach it to the posting by clicking the little paperclip icon in the tool bar (when you're composing the post).
Hope to hear it soon,
Joe J Thomas
Joe J. Thomas Acting Portfolio