You can try with zypper ar -r http://yum.insynchq.com/fedora/21/ but I think there’s some problem right now with the repository configuration that it’s not allowing connections for the moment.
Another option is to create a /etc/zypp/repos.d/insync.repo file with the following content:
[insync]
name=insync repo
enabled=1
autorefresh=0
baseurl=http://yum.insynchq.com/fedora/21/
type=rpm-md
priority=105
gpgcheck=1
gpgkey=https://d2t3ff60b2tol4.cloudfront.net/repomd.xml.key