You have to be logged in with a useraccount on the xerte website to be able to download the zip. The only way to directly get the files is through github and git clone the files.
You can confiure Xerte through https://NETHSERVER/xerte/management.php
default user is admin
default password is admin (or at least it is with the implementation on Karoshi server, where I run Xerte on)
Change the default password ASAP (especially if your server is internet facing)
/edit: Question to @mrmarkuz Why install in /usr/share and not create a virtualhost and go from there? Then create the xerte vhost and unzip to /var/lib/nethserver/vhost/xerte
/edit2: Reading the notes in the install screen, the default dir for a xerte install is /usr/share… would have been nice though, if it could be installed the ‘nethserver’ way.