You don't need to use Carstockalypse or Mapstockalypse. It is very easy to do manually, all you're doing is copying and pasting text.
Rename your DATA.TWT to DATA.TWTbak so game won't use it
Unzip data.zip into your 'Carmageddon 2 Carpocalypse Now' folder
Adding Cars Manually
Open the .C2C that came with the addoncar with a plain text editor. Copy what's in it.
Open OPPONENT.TXT and paste what was in the .C2C file at the end of the file right above the word 'END'
If you're adding a car, go back to the top of OPPONENT.TXT and increase the number by 1. If you're replacing an existing car and not adding to the total number, leave the number at top alone.
Maps are handled similarly.
Open .C2T file that came with the map with a plain text editor. Copy what's in it.
Open RACES.TXT and paste over an existing level with what you copied. (you can't add to the nmber of levels, you must replace an existing one)
ALWAYS BACKUP YOUR OPPONENT.TXT or RACES.TXT before you edit it.