Cracking 1: Wallace says this when you give him the egg in Act 1. You can't miss this one.
Cracking 2: In Act 2 with Wallace, as soon as you can leave the cellar go to the kitchen and flick the egg hammer setting (next to the door) to Tap. He will then say something about walnuts and use the word "cracking". If it's already set to Tap you need to flip it to Bash then back to Tap.
Cracking 3: At the start of Act 3 with Gromit pick up the voice recorder and frying pan from the kitchen then leave the kitchen via the hall. Pick up the map and remote from Wallace's clothes. (Wallace gets the map from Mr Paneer in the Town centre in Act 2 so make sure you have it). Go to the Town centre, show Mrs Gabberly in the newsagent the voice recorder then set the frying pan up on the canopy pole. Go to Mr Paneer's store and shake the pipe to wet the bees. Then return home and go to the kitchen, talk to Wallace, then talk to him again at the blender and he will say "cracking".
NOTE: I have only got him to say this if I have only cleared the town, he doesn't appear to say this phrase if you have done the garden or cellar as well as the town so make sure you do the town first!
Cracking 4: At the end of Act 4 Wallace says this after the tunnel chase. You can't miss this one.