It appears that installation instruction at https://premium.wpmudev.org/project/membership/installation/ are containing some of instruction for old plugin. this text doesn't flow nicely with rest of content.
Depending on your WP install, you can place the plugin files in one of two locations. Standard WP / WPMU (for blog by blog access) Upload the membership folder and all it contents to /wp-content/plugins folder on your site
The path to the main plugin file is wp-content/plugins/membership/membershippremium.php
Please update if i am correct.
