Try this for starters:
;Artificial Intelligence Script
;Header
desc =
;Triggers
:state=0,plrwithinzone=1:state=1,activateifused=1
:state=1,plrwithinzone=0:state=2,activateifused=0
;End of Script
:Condition:Action
Two colon to seperate the condition from action statements.
Also Plrinzone is not a valid condition or statement.
{Edit}
Anyway it does not work like you want. Yes it spawns it, but when you step out of the zone, will you ever need it on again??
If not the you could just detroy it in state=1 line of the controlspawn.fpi script. and it kind of works.
I tried to have its lives at 10 and spawn when dead to be there for reentry into the trigger zone but, the item either would not respawn or did all 10 lives so fast, that it made it seem not to work. I could not tell.
Is this a stock entity? Do you have a model program you could do a simple animation in. Hiding the Object under the floor level when you did not want to see it and having it above the floor level when you did. Giving you 2 anim to be able to call in your script.
Other thing you could try is alttexture action. Your other texture would be all black, so it would be invisible.