Making it appear: if it is already on the layout just set the opacity to 100 or visibility to "visible".
If not, just create the object and use the tween behavior.
Make sure you use the "tween 2 properties" action
+ Keyboard: On Space pressed
-> System: Create object enemy on layer 0 at (0, 0)
-> enemy: Tween "" property Position to 10, 10 in 0.5 seconds (Default, destroy: No)
Alternatively: use timelines, but that's probably overkill here.
If it's not necessary to do it in exactly 0.5 seconds you can also use MoveTo, that depends on the speed you set the MoveTo behavior. There are many examples in my playlist of using the MoveTo behavior.
hope that helps