In the KS Level Editor, open the 3rd Party Tools tab and click the Knytt Stories Manager (Current Screen). next, click the blank box above Shift A circle. click it, and click Sign A. Then, right to it, there is this long box. type in what you want to say in the sign.
E.g "l0l n00b"
EDIT: for beginning cutscenes, choose World Properties and then there should be a tab called Cutscene Editor.