Head straight for the elephant house when the zoo opens, and go into the basement where the pangolins are displayed; there will be boards telling you when the elephant bathing time is (another thing you will not want to miss) and the pangolin feeding time - at this time the lights are turned on and the pangolins are enticed out for food and also weighed by the keepers.
I have written about my experience seeing pangolin at Leipzig
here. As a general rule the Pangolin feed tends to be at around 10.45am, with the elephant bath an hour earlier, but it is best to head straight to the elephant house to check the timing yourself - just make sure you return to the house for the feed/bath about 10 minutes early, as it can get crowded.