Grand company chocobo help

So i finally got enough notes to buy the insurance and turned in my papers and named my chocobo. now it's time to ride alright, cool. WRONG. Can't figure out how to call the damn thing. it gets put back in the stables in gridania and then you can't call it in a city so I leave the city and my whistle is greyed out, and everytime I try to use it, it says "cannot execute at this time". Can anyone point me in the right direction I feel really dense and I'm sure once I get it I'll be like DUH!!!! But for now it's just really frustrating and annoying. Someone anyone, please help.

Is there an item in your inventory to use first before you can use the whistle to call the chocobo?

As said above - there is an item in inventory you've purchased form grand company, use it - it will add the action under Mounts - then map it to hotbar.