You need to think of this as two separate projects - {A} making your motors do things and {B} receiving a message using Bluetooth.
I suggest you write a simple program to receive the message and display it on the Arduino Serial Monitor without worrying about the motors for the moment. The message could be as simple as <123, -79> which would tell the Arduino to make one motor move clockwise with a PWM value of 234 and the other move counter-clockwise with a PWM value of 79.
I don't have any experience with Apple phones so I can't help with the BLE connections but the data should be same whether it comes from an Apple or Android phone.
...R
Serial Input Basics - simple reliable ways to receive data.