Change to :
Var6 =6
VarName6 =kevin magnussen
VarName6 Icon =%iconpath%\denmark.ico or dk.ico ( depending how it is in the file folder into the carset )
VarPreview6 =Files\Previews\Drivers\magnussen.jpg
By the way , magnussen is the var10 rather than var6 .
This will change only in the CSM menu , for changing ingame you have to modify the performance file and also the helmets.ini otherwise the Magnussen's helmet won't match with the car and name .