[Konstantinos]

pawn Код:

// get the IP first
new 
    sz_Ip[ 16 ]
;
GetPlayerIp( playerid, sz_Ip, 16 );

// create the file and write the ip into the file:
INI_WriteString( File, "IP", sz_Ip );

Ip is not an integer but a string.

EDIT: Replace the INI_WriteInt with INI_WriteString too.