Hey rpazos,
Thanks for getting in touch with us. The MailChimp widget for the plugin should show up normally within both parent sites and subsites. The important things to keep in mind when setting things up there, though, are that each separate WordPress installation (if you have multiple separate installations of WP on your server) would need its own separate installation of our plugin, and would need to be separately authenticated by logging in to your MailChimp account through the plugin’s preferences separately per WP installation.
If the plugin is independently installed in your main site and subsites, and the connection is configured independently for each in the MailChimp Setup screen under Settings in WP, and you’re still not seeing the widget appear in the Available Widgets page under Appearance, please update us with what version of the plugin you currently have installed and active in your main site (in the Installed Plugins page) as well as what versions (same or different) you have installed in the subdomains that are working.
Thanks
-Gabe