Adding a logout option to your site navigation is not included in the core functionality for WishList Member.
There is a Logout option that does appear in the WishList Member widget when a member is logged in. The WishList Member widget is located in the Appearance > Widgets section of WordPress.
Once logged in, a member will see the option to logout.
You can also look into the available plugins that offer logout options that can be found in the WordPress Plugins Directory.
Please note that we cannot support these plugins as we did not develop them but they may offer the desired functionality.
Here are some search results from the WordPress Plugins Directory that might help.