I'm making a platformer with a side scrolling script. I want to add enemies into the game but I don't know how to make them show up once the player goes to where the enemy is.

It depends on what kind of script you are using. Usually this would be done by making the enemies appear when a ScrollX variable is in the right range. If you want more detail you may have to share the project and link so we can look at it.

Link: https://scratch.mit.edu/projects/261020891/

You can give the koopatrooper a variable called koopatrooperX, I would do for this sprite only so you can add clones later and they can each have their own copy of that variable. Then tell the sprite to go to scrollX + koopatrooperX.

For hiding off screen you can use the absolute value of the koopatrooper's x position, not its variable position or its scrollX. Use its stage x position. If the abs of xpos is > 230 hide, else show

Inside your koopatroopa sprite, you can use this script:

when green flag clicked
go to x: (199) y: (-114)
forever
if <<(-740)>(ScrollX)> or <(ScrollX)>(-240)>> then // Check if the player is in range
 show
 set x to ((ScrollX)+(480))
else
 hide
 wait (0.1) secs // Optional, might be better for perfomance
end
end

You might want to create a broadcast scroll that runs what is inside the forever loop and only broadcasts when the player is moving so that you aren't unnecessarily running loops.

Do you know how I could modify the script so that the Koopa Troopa moves?

You'll need to do something like this. I haven't tested it so it may need tweaking.

when green flag clicked
go to x: (199) y: (-114)
forever
if <<((thisX)+(250))>(ScrollX)> and <(ScrollX)>((thisX)-(250))>> then // Check if the player is in range
 show
 set x to ((ScrollX)-(thisX))
else
 hide
 wait (0.1) secs
end
end