This can happen on a Mac when the screen isn't maximized. Go to the Window menu and click Zoom. This configures the size of the screen to your monitor and fits it properly so that it will scroll when you reach the bottom of the page.
Final Draft 10 has been engineered to automatically move the text up on the screen when you are typing at the bottom of a page. Information on upgrading is HERE.
NOTE: All support for Final Draft 7, including software activation, has been discontinued. You may find solutions to problems in the online knowledge base or in the PDF manual found on the installation CD.