For some very odd reason, boxee did not include an eject button in their version of the app. Luckily it is pretty easy to add your own eject button. Well, looking back it is easy; however, it took me a while to figure it out...
First off, find a nice fancy eject button and place it into boxee's skin images folder, "skin/boxee/media/icons/". I used this one and found it on google image search.
Next insert these lines after line number 231 of the file skin/boxee/720p/boxee_main_menu.xml
<item> <visible>System.HasMediadvd</visible> <controlid>1300</controlid> <thumb>icons/eject.png</thumb> <label>Eject DVD</label> <onclick>XBMC.EjectTray()</onclick> </item>
And you're set. You should now have an eject button for your DVDs.