Version 1.3 beta.
The option button is available as soon as "Export" is selected from the File menu.

Click "File".
Click "Export". (opens "Export File" window).
In "Save as type" box (select mp3).
Select "Options". (shows "Specify mp3 Options" window).
Select bitrate from "Quality" drop down list.
Click OK.
Click save on "Export File" window.


Also if you look at the bottom left corner of the window, you should see Project Rate (HZ) clcik on the drop down icon to change.

Hope that is what you are looking for.