The Counter-Strike: Source content pack can be found in the Mod Manager, follow these steps below to mount CSS -
1. Installing it within the Mod Manager - simply open the Mod Manager on the control panel, you should then find CSS as the first mod, hit install.
2. Once installed you should see the following popup appear -
3. You will then need to open the Configuration Files section within the control panel, then select the 'mount.cfg' file and hit [ Text Editor ] -
4. Once opened, simply edit your config file to look like this -
5. Save the file and you should be good to go!