Red Jansored Chainsaw

Put the .vpk file in:

64-bit OS: C:\Program Files (x86)\Steam\steamapps\common\left 4 dead 2\left4dead2\addons

32-bit OS: C:\Program Files\Steam\steamapps\common\left 4 dead 2\left4dead2\addons

and when you've done that dubble click the .vpk then you're done!