By default I believe it’s ctrl+space. You can check by going to Preferences > General > Keys and filtering for ‘content assist’.
If it’s not working, you may have a binding from OS X or another application overriding it. If a window with a blank list is popping up, you may need to adjust which suggestions are shown by default in Preferences > Java > Editor > Content Assist > Advanced. You can continue pressing the binding to cycle through the different types of suggestions.