You need the sysctls
$ sysctl net.inet.carp.allow net.inet.carp.allow=1
$ sysctl net.inet.carp net.inet.carp.allow=1 net.inet.carp.preempt=0 net.inet.carp.log=0 net.inet.carp.arpbalance=0
to let the magic work, we need
$ sudo sysctl net.inet.carp.preempt=1