It's a two-part setup in all versions of mIRC. First, you set up the actual command to op or voice a user upon entry into the channel (done in the Remote). Then you define addresses that you want to op/voice in your Users section.
1. In Your mIRC Remote Section (Press Alt-R)
; The below line gives operator status to users on IRC in channel #anandtech. Swap channel name as needed.
on 100:JOIN:#anandtech:/mode #anandtech +o $nick
; The below line gives voiced status to users on IRC in channel #anandtech. Swap channel name as needed.
on 101:JOIN:#anandtech:/mode #anandtech +v $nick
2. In Your mIRC Users Section (Find the Users tab while viewing the Remote)
; Addresses are defined as nickname!identd@ipaddress. Wildcards for any part are accepted.
; Add the addresses of all users you wish to op with a 100: prefix.
100:*!*sully@192.168.0.1
100:*!*anand@24.157.191.*
; Add the addresses of all users you wish to voice with a 101: prefix.
101:*!*baffled@191.48.68.82
Drop me a PM and I'll drop into your IRC channel to help out if needed.
