one thing though, is different at my script. instead of "1" for opening a pop up on first monitor i used 49, since this is the number returned by the SSMGetScreen function when converted to integer. i did this because c compiler reported an error when using "1". Look:
#include "apdefap.h"

int gscAction( void )
{

BOOL bTrigger = GetTagBit("OpenPopUp");

#define PicNameInAction "PoP_CIP_EVAP_AC_END.pdl"

if (bTrigger== TRUE)
{
SSMOpenTopField("1", PicNameInAction);
}
return 0;
}

and error is
line 12: error (0069): type mismatch in argument 1.

so, to avoid this, I use 49 to open on monitor 1 and 50 to open on monitor 2. this the numbers I get when do this:

SetTagWord("Evap_ACret_Monitor",SSMGetScreen(lpszPictureName));//Return-Type: BOOL

so in Evap_ACret_Monitor (integer) is stored 49 or 50 depending on monitor.

and it still works only on the server.

Please, any help?

to finalize this conversation with myself, this thing finally works, this way:
since i don't need pop-ups on the server, i've deleted script on the server. then i've created the same script on the clients, in actions folder (not in global actions) which goes like this:

#include "apdefap.h"

int gscAction( void )
{

BOOL bTrigger = GetTagBit("OS_SV01::OpenPopUp");//Return-Type: BOOL

if (bTrigger== TRUE)
{
SSMOpenTopField(49,"OS_SV01::PoP_CIP_EVAP_AC_END.Pdl");//Return-Type: BOOL
}
return 0;
}

it didn't work until i put the server prefix OS_SV01:: as cabreytjsuggested(everywhere, also in triger tag of this action (which is OS_SV01::OpenPopUp).
so, this opens pop up PoP_CIP_EVAP_AC_END.pdl on the first monitor and the pop up stays up until operator close it.

Where do I start writing this script? I have several OCs connected to several OSs. The script would better be put in the OS no?

As stated above, you should open popups and create scripts on client machines where you need them. This you use only if you want to open popup regardless of current base screen. In any other case, if you want to open popup which exists only in relation to particular base screen, you should use picture window object.

regards.