The NavigationLayouts module you downloaded was from Mendix 6, so I think you actually took a step backwards when you downloaded it on top of Mendix 7.8 (which uses the newer UI framework, AtlasUI).
The errors you see are easily solvable, however, if you really wish to use it. Double-click on the error, and it'll bring you to the page with the problem. In this case, the error is that a page is trying to use a layout that no longer exists. To fix this, look at the "Properties" pane in the modeler and find the "page layout" property. Change the page's layout and you'll be all set. (You can also right-click on the page in the project explorer and choose properties to see the page layout setting).