Use /ignore -r to remove the ignore on someone see /help /ignore.
Alternatively and you might find this more useful, you can hit alt + b in mIRC, which will bring up the address book, go to the Control tab, select ignore (should be there by default), and you can remove ignore from there.
That didn't seem to be the issue.
I just read in another thread at the Forum that the Channel List Modes were disabled in the latest update.
That kinda sucks, hope they'll consider bringing it back in the next update..