AIX:
1. Edit /etc/inetd.conf and uncomment or add entry for rstatd:

root@aix~# grep rstatd /etc/inetd.conf
rstatd  sunrpc_udp      udp     wait    root    /usr/sbin/rpc.rstatd    rstatd 100001 1-3

2. Edit /etc/services by uncomment or add entry for rstatd:

root@aix~# grep rstatd /etc/services
rstatd          100001/udp

3. Refresh services:

root@aix~# refresh -s inetd 
0513-095 The request for subsystem refresh was completed successfully.

4. Check it:

root@aix~# rpcinfo -p | grep rstatd
    100001    1   udp  32822  rstatd
    100001    2   udp  32822  rstatd
    100001    3   udp  32822  rstatd

Solaris:
root@solaris~# svcadm enable rstat