With Multistore networks, installation of plugins and themes is handled at the Network-level rather than at the site level.
Navigate to My Site > Network Admin > Plugins. You will see the plugins that have been installed in the network. Keep in mind that not all plugins are activated on all sites in the network,
You can install new plugins by click the button "Add New" and you can update existing plugins by clicking the link "Update"
Navigate to My Site > Network Admin > Themes. You will see the themes that have been installed in the network. Keep in mind that not all themes are activated on all sites in the network,
You can install new plugins by click the button "Add New" and you can update existing themes by clicking the link "Update"