Do you have all the php extensions needed and openssl installed? As root user from terminal:
php -m
openssl version
Are the two moodle instances you are trying to mnet on the same host? or on a different host? Have you set up both instances with peer settings:
http://docs.moodle.org/24/en/MNet
What do you get when using command line curl for information:
from MoodleB
curl -I http://someserver/moodleA
from Moodle A
curl -I http://someserver/MoodleB
And since this is on CentOS ... have you looked at any logs? apache error log, secure log, messages log?
What do you get using:
http://moodleA/mnet/publickey.php
http://moodleB/ment/publickey.php
'spirit of sharing', Ken