Laxmidi:
You wouldn’t FTP to your local installation of WordPress, which is under some directory that you have assigned with MAMP.
Wherever that directory lives, you must unpack your theme.zip folder on your desktop, then move the theme into /wordpress/wp-content/themes
Once you add move your downloaded theme directory into WordPress, you should see the Theme under Dashboard -> Appearance -> Themes, after which you will activate it, then navigate to the local URL and see your WP install working.
All files should be 644 and all folders 755.
Tony Zeoli