1. Place folder inside the models directory 

(left 4 dead\left 4 dead\materials\models)

2. Open up pak01_dir.vpk and change the line 

($baseTexture "models\survivors\biker\francis_color") 

to 

($baseTexture "models\survivors\biker\srancis_color")

3. Save

4. Play

Always back up your pak01_dir.vpk file