You have to add the scuffs yourself. That's the tricky part. I'd suggest doing them on separate layers so as not to ruin the originals.
EDIT: Let me elaborate. This post was too short.
For small scuffs and scratches you can use a narrow pencil tool. For larger ones, some use a type of pattern brush (I find this doesn't look good, as the repeating pattern is quite obvious). You can use larger paint brushes with soft edge and blur them. All of these can be coupled with reduced opacity and different blending options. This is just too open-ended a question to answer.
Can you be more specific? Show a screenshot of the area you want to scuff, and what you've got so far?
