The main navigation in inCMS is always horizontal, lower levels (if any) are either vertically displayed as submenu in the sidebar or as a drop-down menu (depending on your settings).

If you want the main menu to be vertical instead of horizontal, use this little trick:

Create a new page in inCMS. As this page is only intended to serve as an invisible "place holder", make sure that "not in menu" is checked in the page properties. 

Give this page a short page name as the page name will be visible in the URL later. In our example, we call this page "menu". You can of course use any term.

Next, drag & drop all pages below the place holder page.

Please make sure that a multiple-column-layout is active on all pages (at least 2 columns), so that the vertical menu can show up. You can set the layout in the page properties of each page.

Here are two screenshots:

Vertical menu in the inCMS backend: 

Vertical menu displayed on the website: