Hi there,
you don't have to use the amibroker developer kit, those are for different purposes.
to develop indicator, you use their special language call AFL, (you can download their user manual, or just download the trial versioni of the software, and look at the user guide) stands for Amibroker Formula Language, take a look whether it is for u, because Amibroker also let you backtest your strategies, so if you can handle the AFL, then it might meet your requirement.
To develop your own indicator with package software, you will most likely have to use their language and do some "programming", so if you do not feel comfortable having others develop the indicator for you (because you prefer to guard your indicator secret), them u will have to learn to program some system one way or another.
good luck....