For SLE 11 SP1 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:ftp/SLE_11_SP1/server:ftp.repo
zypper refresh
zypper install proftpd
For SLE 11 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:ftp/SLE_11/server:ftp.repo
zypper refresh
zypper install proftpd
For SLE 10 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:ftp/SLE_10/server:ftp.repo
zypper refresh
zypper install proftpd