Dreamweaver CS4 does not support HTML5 tags however, you may add this functionality yourself fairly easily.

To do this go to Edit > Tag Libraries

Click the + button on the top right and select New Tag.

The select the tag library: HTML Tags and enter the tag you wish to add there.

Tip: You may enter multiple tags at once by separating them with commas like this:

article, aside, audio, canvas, command, datagrid, datalist, details, dialog, figure, footer, header, hgroup, mark, meter, nav, output, progress, rp, rt, section, source, time, video