The instructions are in the readme:
Use SparkIV or OpenIV or anything that can open img files,then replace
in "GTA-IV\Grand Theft Auto IV\pc\data\cdimages\scripttxds.img",save the changes and close the program and enjoy the theme!
So download and install SparkIV (if you haven't already) just google it
Then open SparkIV and hit the big 'browse' button and go to pc/data/cdimages and double click the scripttxd.img. Now hit the big 'import' button and select the files that came in that .rar
Now all the files you replaced should be highlighted in blue, click save and then exit sparkIV. You might want to backup the files you are going to replace if you haven't already got a backup for gta4.