A box will come up showing the menu items on your website. Your main website links will be under 'mainmenu'. Just click the page in that menu tree you want to link to and click insert.
If you see the page there you want to link to, highlight the page title in that list and click insert at the bottom right.
If you do not see the page you want to link to there, click 'articles' or 'static content' at the top to see all the pages on your website even if they are NOT in the menu system.