Note: Bob's Buddy is currently only available for Hearthstone Deck Tracker for PC. We hope to extend this feature in the future to HSTracker for Mac and Arcane Tracker for Android, but we don't know yet when or if this will happen.
First of all, double-check that you have the latest version of HDT installed on your PC.
In Options, navigate to Overlay > Battlegrounds, and you'll find Bob's Buddy there! Just click the "Enable combat simulations" and "Show results during combat phase" options there to enable Bob's Buddy. The next time you enter combat in Battlegrounds, you should see your likelihood to win, lose, and tie at the top of your screen.