The Contact Us addon became a standard feature of the software in version 5.0.1. On new v5 installations all you need to do is install and enable the addon, and it will display on the "contact us" extra page. If you update to v5, after you install/update the addon in the admin panel, you will need to put the following tag where you want the contact us form to appear:

{addon author='geo_addons' addon='contact_us' tag='contact_form'}