you define a vector, set it to the mouse x and y and the z to the distance, where the particles will be spawned. then you pass it to the vec_for_screen function and at the last step you use it as the spawn position in effect().

Code:
VECTOR prtPos;
vec_set(prtPos, vector(mouse_cursor.x, mouse_cursor.y, 50));
vec_for_screen(prtPos,. camera);
effect(yourFunc, 123, prtPos, nullvector);


hope this helps =)
Scorpion