WP Estate is compatible with dsIDXpress and Optima Express.
IDX plugin compatibility means that the theme works well if you add 3rd party plugins (such as dsIDXpress or Optima Express). The theme will not return any errors just by using these extra options. IDX plugins are Optional and they are not required for the theme to be installed and used.
How IDX works: If you install one of IDX plugins, they will work with the theme, without conflicts. BUT the properties added through the plugin can be managed only with plugin features, options. They will NOT be used with the theme options. The properties from IDX are managed in a different database and the theme cannot access those properties (the plugin developers do not offer such access). So you use the plugin as it is, with its own design and features.