First I advise you to rename the htaccess.txt file since you are using SEF (rename to .htaccess
To add a page to a menu is joomla basic:
Go to Menus, find Main menu in the drop down.
Click on the Testimonials and make sure it is Article in Menu type..
If you do not have it as single article layout, then choose it, and then in the parameters, select this article.
you have error on all your menu links