Ryan9295

Well, you are going to need a few things. First the application OpenIV, this will give you access to those "hidden" files. You will also need this application to install asi manager and OpenIV.asi
So lets start there. Download OpenIV ( http://openiv.com/ ) After that's installed, you will be able to select what game you need. Obviously it will be GTA V windows version. Then it should ask you to navigate to where you gta.exe file is. Since you have a retail copy, it should be Documents>Rockstar Games>GTA V, (I have the steam copy, so I am not entirely sure where its located for you) Once you find it, you can begin. Start by creating a folder in your directly titled "mods" then locate the "update" folder. Copy and paste that into the "mods" folder you just created. (This is where your cars will go. You can skip the mods folder step and install them directly in the update folder, but with a mods folder you can easily utilize GTA online by just removing a single file instead of having to replace all the cars you created. If you do join online with custom cars, you will be banned.) 
Next, go to the tool bar in OpenIV and click on the box with an arrow in it... I attached a photo of where its located. You'll see two options, install them both. This is what points to the mods folder. Simply un-install both of these to default back to the vanilla Update folder. 
Now we can install some vehicles. This is easy once you get the hang of it. Every car, plugin should have a Readme.txt file included in the download. Inside that should tell you specifically where to install them. I install all my cars in "patchday3ng", however, I know Police3 needs to go in patchday4ng, so its important to read each readme file.
Find a vehicle in the Modding tab and download it. I'm not sure how much you know about the file system. But just incase, I'll briefly explain. Included in the downloaded model are 4 files with the extension .ytf and .ytd. The .yft files are fragment files (thats the physical structure of the vehicle) and .ytd is the textures of the models. The _hi and +hi are what unloads at a distance if my understanding is correct. Regardless you will need to replace all 4 files. Some might come with 3, just replace them the same way
So now in OpenIV, navigate to mods\update\x64\dlcpacks\patchday3ng\dlc.rpf\x64\levels\gta5\vehicles.rpf\ and hit edit mode at the top and drag and drop. After you are done, I would recommend hitting edit mode again, because misplacing even one file will corrupt your game. So be careful.
I was hoping for a shorter tutorial, but hopefully this helps you. If you have any questions, feel free to ask, or watch some YouTube videos that another user has posted above.
 
Best of luck,
Ryan
 

Clearly you have the latest build of GTA V, I assume you have a Mods folder, and I am also assuming you created that Mods folder a few versions ago. If you did, then you will need to update your mods folder. There are new DLC packs that were added to the vanilla "update" folder, you will need to add those DLC packs in with the Mods folder, or simply re-copy the vanilla "update" folder, and add your vehicle mods again. If this isn't the cause of your errors, then can you supply a little more information? What does the error message say? When does it pop up? Can you still launch the game vanilla?