Matomo plugins can be installed from the Marketplace view. Only the Matomo super-user can install plugins (Matomo admins cannot install plugins).
Please be aware that there is a bug in Matomo that you cannot install plugins when LDAP authentication is enabled.
To workaround this issue:
Deactivate the LDAP plugin.
Install required plugins
Re-enable the LDAP plugin
The matomo UI shows many intermittent error messages on Firefox. We recommend using an alternate browser.