I can't find that option so it must be for the paid version only.
Posted from my CrackBerry at wapforums.crackberry.com
Open the program, go to Setting/Homescreen/Notification Icon/All the way to the bottom there is a check box for Show temperature icon in notification area. If you check that, you get the thermometer.
Don't know if it is dependent on the free or paid as I have the paid. Well worth the $ IMHO.