Have a trigger, such as time, score etc.
Then - System -> Create Object (which is right at the top)
Choose a layer and then input and X and a Y value. X and Y being a pixel value corresponding to the playing area. Just say your layout size is 1280 by 1024 (you can check this by clicking on your layout in the Projects Bar), the enter a number between 0 and 1280 for X and 0 and 1024 for Y.
Instead of entering a number, you can use the "random" expression to make it generate a random number. In which case enter random(0,1281) in the X section, and random(0,1025) in the Y section of the Create Object event. The random() expression will return a random number between the two numbers you enter. Well actually, it will return a value between and including the first number, and one less than the second number, hence why I added 1 to the last number of those examples.