spawn <mobtype> at <location>
location includes X, Y, Z and world.
you can also use "<number> of <mobtype>" to spawn multiple.

for example:

spawn ghast 6 blocks east from event-player's location
spawn 5 zombies the location at x =10 , y = 100 z = -10 in world %world%

Code (Text):

1. [the] (location|position) [at] [(][x[ ][=[ ]]]%number%, [y[ ][=[ ]]]%number%, [and] [z[ ][=[ ]]]%number%[)] [[(in|of) [[the] world]] %world%]
2. (spawn|summon) %number% of %entitytypes% [%directions% %locations%]

 

wait you want to have a command ingame to spawn a zombie at a location?
something like this could be used... i guess.
also don't assume people don't have other stuff to do than help you.

Code (Text):

1. command /spawnmob <mob: entitytype> [<x: number> [<y: number>] <z: number> [<world: world>]]:
2.     executable by: player
3.     trigger:
4.         if arg-2 is not set:
5.             set {_x} to event-player's x-coordinate
6.             set {_z} to event-player's z-coordinate
7.         else:
8.             set {_x} to arg-2
9.             set {_z} to arg-4
10.         if arg-3 is not set:
11.             set {_loc} to the location at {_x}, 1, {_z}
12.             set {_loc} to highest block at {_loc}
13.         else if arg-3 is set:
14.             set {_loc} to the location at {_x}, arg-3, {_z}
15.         if arg-5 is set:
16.             set {_loc}'s world to arg-5
17.             spawn arg-1 at {_loc}
18.         else:
19.             set {_loc}'s world to event-player's world
20.             spawn arg-1 at {_loc}