When you first install MobileCartly in the root of your directory, you should visit http://www.yoursite.com/mobilecartly.

You will be asked to enter in your API KEY, which you can get one (free) at http://www.MobileCartly.com by logging in and adding a new site.

The API KEY form is temporary; Once you add your API KEY and you site is verified, it will disappear.

The API KEY is not a crucial security feature..  it acts more like your store ID.