If I remember correctly, I believe it only works with 4.6 and above. I also don't remember if this is supported on the free version. It may not be. It is supported the paid version for sure though. Also make sure you have downloaded the correct version of BW for your device.
Then, go into Settings-> Homescreen/Notification Icon-> scroll to the bottom, and there should be an option there for "Show Temperature Icon In Notification Area". Check that box, and save your changes. It should show.
Other than that, I guess I can only recommend a battery pull, or a clean reinstall.
HTH.