Copy the downloaded MasterServers.vdf into that folder.

Go to the platform/config/ folder. If a MasterServers.vdf already exists, delete it or back it up.

For Steam users: C:\Program Files (x86)\Steam\steamapps\common\Counter-Strike Source\platform\config For No-Steam/Old builds: [Game Folder]\platform\config

For safe + easy + online play, buy CS:S on Steam (~$10). For legacy/offline only, search for "CS:S v34 master server fix archive.org" – those are generally safe.