Look at the image and pick what type you need

Each folder is a different option.

Copy and paste the dod folder to...

64 bit systems...  
C:\Program Files (x86)\Steam\steamapps\Your-Account-Name\day of defeat source

OR
32 bit systems... 
C:\Program Files\Steam\steamapps\Your-Account-Name\day of defeat source

OR
You have installed in a non standard location... you know where that is then!

INsane

dodbits.com