WishList Member is a WordPress plugin so it needs to be installed onto a hosted site running WordPress in order to function.
WordPress.com does not allow for plugins to be installed so you would need to download WordPress from the WordPress.org site and install that onto a domain that you own.
WordPress.COM and WordPress.ORG offer different options.
You can download WordPress from the WordPress.com site.
You would also require a hosting account (a place to house your site).
Once you have downloaded WordPress and installed that on your domain (housed on your hosting account) you can then install the WishList Member plugin (and any other plugins you would like to install).
If you have any specific questions about downloading or installing WordPress, you can contact their support team.