There are 6 boxes in the spawnprojectile event.
The first boxes are... well the first numbers you enter, if you only enter numbers to these it will always go to the same direction.
If you enter numbers to the first 3 boxes and the second 3 boxes it will pick a number between the entered numbers.
Ex.
[10] [20] [30] --- [20] [30] [40]
The bold (first) numbers from both boxes are for the x(or y?) axis so it's x axis speed will be picked from between 10 and 20 (including 10 and 20).
The second numbers ... and so on.
So this would be your spawnprojectile event.
[20] [20] [10] --- [(now i don't know what it does if you enter 0 here so if it doesn't work with 0 try 20] [(same here)] [40]
Hope it works.