Ok, so there should already be a menu called "Main", this controls the top level items in your portal.
There is another system configured one called "Secondary", which appears on pages you create other than the home page.
To add menu items, just select the menu from the drop down and then add items to it (you will see the drop down expand to show new items)
Select the item and fill in the name and target, you can do this over and over to create your menu structure.
Then save the menus and they should show up in your portal home (Main) or sub pages (Secondary).
All the templates and logic are in the portal/ folder of your Tyk dashboard installation (usually in /opt/tyk-dashboard)