There are two gaps controllers at the top of the panel, where you can define gaps between pages. Simply type the number there (or several numbers if you need a pattern). Imposition Wizard will loop through numbers for each new page in the row or column, so you can group pages by entering “0 10” for example.
Below the gaps controller there is a layout selector which allows you to pick one of the supported layouts, that you can later configure using the details panel at the bottom.
See the layouts section for more details about the layouts.