how to assign more permission to wordpress author

I have not looked at this plugins code, but what you will need to do is locate the part of the code that applies to show only this menu to the admin (normally it looks something like:)

if ( current_user_can( 'manage_options' ) {
// some code stuff in here
}  

form there you should change the manage_options text to upload_files which is a defined capability for author role and above (or you could create a new capability which only applies to author role). Creating a new capability is probably the best option in my opinion as then you could target specifc trusted authors to have permissions to the menu by assigning that new capability to username.

This should do as you require but be warned any update to the plugin by the plugin author will override this so for each time the plugin is updated you will have to re-apply this slight modification.