Lists are not randomized directly but you can get a similar thing by randomizing trials within a block. This works because at run-time, SuperLab converts trials that use events that use lists into multiple trials and multiple events.
To literally see how SuperLab does it, click on the Experiment menu and select Expand Events That Use Lists. This will open a new experiment window that shows you exactly how the conversion from individual stimuli list items to events and trials is done.
Caveat: I recommend that you use the Expand Events feature only for viewing. If you make changes in the newly opened window, these changes will not be reflected in the original experiment that uses lists.