For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.3/16.0/server:Kolab:16.3.repo
zypper refresh
zypper install kolab
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.3/15.6/server:Kolab:16.3.repo
zypper refresh
zypper install kolab
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:Kolab:16.3/15.5/server:Kolab:16.3.repo
zypper refresh
zypper install kolab