Have you tried using modloader for this pack?
Edit: Ok found out where you got the pack from. OT specifically instructs you to do this use this method:
Each model supports both collar pins and rank patches. They are applied via alpha mapping, which makes it a seperate texture in the .txd. This allows for quick editing of the rank pin or patch, which anyone can do. There's a rank pin or patch provided for each rank, but you will have to put it in the .txd yourself using TXDworkshop. The ranks start from PO-3 to Chief of Police, taking along the Sergeant career and the Detective career.
Each model supports both collar pins and shoulder patches. The collar pins are controlled by Pin and Pin_n in the .txd. The shoulder patches are controlled by Patch and Patch_n. The Pin_n and Patch_n images are the normal maps, which are blue. Replace those with the rank_norm images. The regular Pin and Patch images are to be replaced with the normal ranks.
For example, to get the captain bars, you import the Captain.png into the Pin slot. You import the Captain_norm into the Pin_n slot. You don't change the file names of the Pin and Pin_n slot, they stay the same regardless of the rank. For a Sergeant patch, I would import SGTI.png into the Patch slot and SGTI_norm into the Patch_n
So my question would be:
Are you looking in the right place for the files?