In order for this feature to work on your site, you will need to have your portal set to ON in your flexmls account.
You can verify this by going to Preferences>Portal Preferences in flexmls.

The following options have been added under flexmls® IDX>Settings>Portal

OAuth Credentials

When you have activated your flexmls IDX WordPress plugin, the information in this section should automatically be filled in.

If there are any problems, please contact us with your API key or flexmls username at:

Portal Registration Popup

Enable Listing Carts:  Check “

Pages To Show: This option will allow you to select the pages you want the registration/log in window to display on.

When to show: With this setting you will be able to set when your visitors will be asked to register/log-in.

Force users to register/log-in: By checking this setting, it will remove the “Not Now” link that appears in the registration/log-in window. When a user is able to click on “Not Now”, it allows them to continue looking at the listing details without registering or logging in.

Location on page – This option will allow you to choose where on the page you want the registration/log-in to display.

Portal Registration Text – Here you will be able to change the text that displays in the registration/log-in window.