I would first find out where all the coolant went. I changed my coolant in my Toyota 2 years ago and have not had to add a drop in over 25,000 miles of driving. there is a reason why you need to add coolant.
Yes, you should add a 50/50 mix to the reservior when the engine is cool. But only to the line on the bottle marked " full cold". There is head gasket concerns on theses engines, but you will not smell it and cannot see it leaking unless the vehicle is on a lift. Then inspect the left rear corner of engine where the head mates with the block. You will see signs of oil and A/F seaping from that spot. There is a cooling system recall on this concern that is free of charge at a dealer and takes about 20 minutes. A head gasket repair is about $1400 if the recall has not been performed.