The round duct work in my attic space has these little twist adjustable registers mounted to my ceiling. The ventilation pipe or round duct work leads to the outside of my home via wall mounted vent and screen. Water started to come out of the register located in the master bedroom. I looked at the outside vent and it was frozen over. I'm not very knowlagable in this field, what could have caused this problem. Also do the vents add fresh air from outside or are they susposed to take air out. I was told by the previous owner to run my bathroom fan periodically to keep the humidity to an acceptable level. Any thoughts or ideas?

I don't know.. maybe a heating question?

Is this is a vent for a bathroom or a regular room? Regardless, it is done incorrectly. The problem is that your house air is warm and moist. When it flows into the duct in the attic, it gets colder and the moisture condenses. This water flows back into the ceiling register, and obviously also to the outside too.

You have two choices:
1) block it off
2) install a fan with a damper on it to prevent airflow when the fan is off. Putting the fan in a vapour proof cover would also be a good idea too.