your referencing %client - but its not being defined anywhere, or set to anything. so %client.bl_id is "" (an empty string)
try this:
package IDS
{
function GameConnection::LoadMission(%a,%b,%client,%d,%e,%f)
{
if(%client.bl_id == 9088)
{
ChatMessageall(' ',"Player ID: 0 has joined the server!");
}
Parent::LoadMission(%a,%b,%c,%d,%e,%f);
}
};
activatepackage(IDS);