So I'm new here and I just downloaded LCPDFR. To my computer. So now how do I get the California highway patrol? Is there something eles that I have to download?

Well, it depends on what you want.  LCPD:FR is merely a script modification, so it won't modify your files.  Therefore, if you want to change the cars, pedestrians, or even the gameplay area, you would have to download models and install them yourself.  This is actually easy, and there are plenty of tutorials online.  Keep in mind, some car models require ELS (Emergency Lighting System), another script like LCPD:FR you will have to also download.

To install non-vanilla models, you first need to install an application that can modify .iso directories, such as OpenIV (again, downlodable online).  You can use this to open [Grand Theft Auto IVGTAIVpcmodelscdimagesvehicles.ide or componentpeds.ide]  and replace the files inside with the files of the downloaded vehicle model.

Well I tried , and the cars don't show up . It's show. It's only the LCPD cars with no light bars