From the rdump man page:
RSH Dump uses the contents of this variable to determine the name of
the remote shell command to use when doing remote backups (rsh,
ssh etc.). If this variable is not set, rcmd(3) will be used,
but only root will be able to do remote backups.
Basically, you need to set your environment variable RSH to your ssh binary. Somethng like the following should work:
setenv RSH /usr/bin/ssh
Of course, you will need to replace /usr/bin/ssh with the path to ssh, but the one listed should be right for most distro's.