In other words, instead of entering a numerical value into your actions like,

X = 9

You replace it with,

X = random(0,640)

What this does is give you a random number with decimals between 0 and 640. However, this gives you decimals which you don't want.

So you FLOOR this value by making,

X = floor(random(0,640))

So this gives you an integer between 0 and 640.

In my previous post, I mentioned rolling a random number between 1 and 10. I wouldn't need to floor it, but you'd need to use random(0,10)

Since you're spawning these objects at the specific point, don't think you'll be needing to create an object at a random X,Y.

I'd set a DICE variable to random(0,10)

and check if that variable is below 1, if it is, spawn an object at the specific point.