Hello preston,
Aircraft carriers are mobile airfields at sea 
and have limited carrying capacity which can be increased through research. Aircraft carriers can accommodate naval bombers, fighters and tactical bombers (lvl 4+) only.
To equip an aircraft carrier the aircraft has to start from an airfield or another aircraft carrier in range of the carrier you wish to send them to. So they have to arrive/ land flying. They also need to be outside of your harbour.
So if in range just select it, press the "march" option, then select Carrier. I hope this helps you.
