Edit: Re-Wrote Tutorial
1. Locate “Dxsupport.cfg” in your Team Fortress 2/Episode Two/Generic Source Game bin folder.(Default Path: C:\Program Files\Steam\steamapps<account> eam fortress 2\bin) and copy it to your Source SDK bin folder. (Default Path: C:\Program Files\Steam\steamapps<Account>\Source SDK\bin(Orangebox or ep1)\bin) Open the file in notepad (or any other text editing program)
NOTE: If you can’t find it either in your Source SDK bin folder or TF2/Generic Source game folder then download a copy here.
2. Locate your Graphics Card in the file (There may be multiple entries, if so do it for all of them.) If your GPU doesn’t exist within the file, go to the bottom and find either Nvidia Unknown or ATi Unknown.
3. Add or modify under your graphics card entry:
4. Save the file and enjoy Anti-Aliasing (This can also work for other Cvars/console commands.)