If you are unable to type in a mobile document (i.e., you type but nothing appears), please tap on an existing line of the document to ensure that the insertion point is there.

For instance, in a new mobile document, the first line is "FADE IN:". If you tap after that and hit Enter, you will begin a new line.

(There is a currently a grayed-out, non-editable box that appears at the end of a mobile document. This is a known issue.)