This download includes every Population file that can be currently found in the game. Population files are used to determine the different robots and waves in the Mann versus Machine gamemode.

If you want to extract the files yourself, first download GCFScape ( http://nemesis.thewavelength.net/?p=26 ). This software will allow you to open VPK files, which are basically like ZIP archives.
Now, navigate to your installation of TF2. This can easily be done by right clicking the game in Steam and clicking Properties -> Local Files -> Browse Local Files...
Here, open the tf folder and look for the file tf2_misc_dir.vpk. Open that using GCFScape. A new window will appear, which will allow you to browse the different files inside the VPK. This VPK in particular is mostly storing text-based files.
Now doubleclick the folder scripts and inside that population and there you have it!

When making your own Population files, store them directly in tf/scripts/population or tf/custom/[anyfoldername]/scripts/population. All these filepaths are considered identical to the game. The only difference is the order in which they load, causing things to override if neccessary.