1. You have to change it where the horizontal menu is in the theme – not being familar with that theme I assume it is in the header.php.
2. It depends what you mean by guests… If you raise the level of registered users to “contributor” or whatever you want (in the admin > Users) then they can write articles and upload files.
Please, be aware it is a risky thing, so give such capabilities only to people that you trust!
To fine tune the roles and capabilties of the registered users, you can install the Role Manager plugin:
https://redalt.com/wiki/Role+Manager
