For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/games:openra/openSUSE_Tumbleweed/games:openra.repo
zypper refresh
zypper install mono-nat
For openSUSE 13.2 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/games:openra/openSUSE_13.2/games:openra.repo
zypper refresh
zypper install mono-nat
For openSUSE 13.1 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/games:openra/openSUSE_13.1/games:openra.repo
zypper refresh
zypper install mono-nat