pmurph, don't worry it can take a while to get used to this stuff. I'm still learning new things, and asking silly questions all of the time :) I've added some steps below.
1. Add a 'Compare two values' event (found in the general section of the system events).
2. Click on the first value, and in the Objects expressions window select system. This will give you a whole bunch of options. Scroll down to the maths section, and you'll find the distance option.
3. Click on the distance expression (this will add it to your comparison equation).
4. To add the enemy & turret X & Y values, use the object expressions window again. click on either object (enemy or turret), and scroll down until you find the expressions for each items x&y. Add x&y values to the equation (for both objects).
5. Once you've added the distance + x&y values, click on the second value box. Here you'll need to add a comparison value for your equation. So for example, if you want the turret to recognise an enemy when it gets within 50px you'd add 50 in this box.
Hope that helps to get you going in the right direction.
Good luck :)