As per the readme document inside the rar archive (you need winrar to open it)
"// G17 Media
// Traffic Flow Mod
// GTAIV
// ---------------------------------------------------
// TRAFFIC FLOW README
// - V1 (April 2011)
// - © Copyright 2009-2010, G17 Media
// ---------------------------------------------------
Traffic Flow V5 is a simple script for GTA IV that updates the game's memory every 15 seconds
with vehicles of your choice which helps to provide more diverse traffic within GTA IV.
To change the vehicles which Traffic Flow should load to the game's memory - open the
TrafficFlow.ini configuration settings and read the instructions contained withih.
To install TrafficFlow V5, make sure you have an ASI loader, the C++ Scripthook and
an up-to-date version of the .NET Scripthook. Simply copy over TrafficFlow.net.dll
and TrafficFlow.ini to your /scripts folder in your GTA IV or EFLC directory.
TrafficFlow operates automatically and does not require any manual action once
in-game."
ie, drag the .dll file and the .ini files from the rar archive to this folder:
gtaivscripts