Make sure you have a button on the page, and that it is inside <form></form> tags.
DW Menu > Insert > Form > Button > Give it an ID and click OK > You should get a message window asking if you want to add a form tak. Click 'Yes', and save the page.
The form/button in code-view should look something like:
<form id="form1" name="form1" method="post" action="">
<input type="submit" name="download" id="download" value="Submit" />
You should now have the option to select (in this example) 'Button: Download pressed'.