By default only certain roles can upload files. If you find this in Web.config:
<add key="RolesThatCanUploadAndBrowse" value="Admins;Content Administrators;Content Publishers;Content Authors;Store Managers" />
Copy and paste it into user.config and add any roles you want separated by semi colons. Save it, then type a space in Web.config and save it so that it picks up your changes.
Joe