Like this
SYSTEM - CREATE OBJECT(SHIP) at floor(random(0,640),random(0,480))
This will create the ship you want to spawn at any location across or down the screen. The (0,640) tells Construct to pick a random number from 0 to 640 (saying your screen is 640 across, you obv change that number if your screen is smaller or larger). The same for the Y coord.
When you get the section that you normally would just enter the X and Y coords of a created item you just type in what I wrote above. FLOOR and RANDOM are not something you can click to select, you must enter them manually.
When Construct creates a random number it's a floating point number so it would be something like 1.23 for example. That's no good to the coordinates so you use FLOOR to round the number down to the closest Integer. So 1.23 would round down to 1. Hope this helps.