1) you can add new pages by clicking on the "new document" button in the site menu in the manager. If you want to add sub pages you can use the contextual menu by clicking on the document icon in the doc tree (in the manager) and select ’add page here’. Here is a tutorial for adding new pages: http://www.modxhost.com/index.php?q=8.html
2) Snippets are made up of the logic code and the snippet call (which places the snippet on the page). Each snippet has a different setup but most come with clear install instructions. http://modxcms.com/adding-snippets.html
