To add a list to a folder first click 'Audience' in the main navigation bar then select 'Lists.'
Next, click the triple-dot menu to the right of each existing list that you'd like to add to a folder.
Select 'add to folder' from the drop down or create a new folder.
You can also create a new folder by clicking the blue folder button in the upper righthand corner.
Keep in mind that lists work just like any documents you have on your computer. They can only be stored in one folder at a time. However, you can always duplicate your list and store them in multiple folders.