Momentum Mod is a free standalone Source Engine modification that brings surf and bunnyhopping gamemodes from Counter-Strike: Source with a variety of quality of life improvements. At the time of writing it's still in an early stage but the mod shows great potential. You can even play on existing maps, but that's where you might run into some issues.
The Source SDK 2013 base doesn't include any files from Counter-Strike: Source. The people behind Momentum Mod can't include this in their releases either due to copyright issues. So what can you do if you don't own or have any plans of owning Counter-Strike: Source? Well, it's not actually as bad as it sounds.
Firstly, we're going to need Steamcmd - it's sort of a command-line version of Steam which can be used to download almost anything from Steam as long as you have the permissions to do so. Fortunately, the Counter-Strike: Source dedicated server is available for anyone to download, and you guessed it, it includes all the files we need to get our maps looking as they should.
Valve's page on Steamcmd has detailed instructions for installing and usage so I'll assume you've already downloaded Steamcmd, opened it and you're waiting at the Steam> prompt.
First, log in anonymously. You can log in with your actual Steam account but it's completely unnecessary.
Steam>login anonymous Connecting anonymously to Steam Public...Logged in OK Waiting for license info...OK
Now we'll tell Steamcmd to download the dedicated server files to the 'tmp' folder.
Finally, we can start the download, this might take a while depending on your connection speed.
Steam>app_update 232330 validate Update state (0x3) reconfiguring, progress: 0.00 (0 / 0) Update state (0x61) downloading, progress: 97.37 (2250551809 / 2311233673) Update state (0x81) committing, progress: 70.83 (1637151855 / 2311233673) Success! App '232330' fully installed.
Time to extract the files. Currently everything is packed into VPK files so we'll need a tool to extract them, luckily we don't have to look far because Valve graciously include their own program with every Source game. Open a Terminal or Command Prompt window and change directory to the 'tmp' folder we just used downloaded everything into.
Tip: Hold Ctrl+Shift then right click on the 'tmp' folder and select "Open command window here".
If you're on Linux, run './bin/vpk_linux32' instead. Should be something similar on Mac OS X but I haven't tested it. Anyway, now we've extracted everything there should be a folder called 'cstrike_pak_dir' in 'tmp/cstrike' now. Open it and copy the materials, models and sound folder to your Momentum folder. Restart the game and voila!
That's all! You can safely remove the remaining files if you don't plan on hosting a Counter-Strike: Source dedicated server.