Hi there!

I just downloaded a vehicle pack (called Sheriff pack) and the cars dont show up in game... I checked the ELS xml files, the dlc list and if I actually copied the Sheriff Pack into the pack_default. Where is the problem?

I hope you can help me!

It sounds like you're trying to install an addon pack
I would double check a few things and make sure they're all in the right location 
First in OpenIV I would go to mods\update\x64\dlcpacks and make sure that the folder that came with the vehicle pack is located there. It should be a folder with a dlc.rpf inside of it named something like sheriffpack (or whatever the pack is named, this changes between packs)
Second I would go to mods\update\update.rpf\common\data and open up the dlclist.xml 
There should be a line in that file that looks something like this:

 

<Item>dlcpacks:/sheriffpack/</Item>

I'm using the placeholder name of sheriffpack for simplicity but the named in between those / should match the name of the folder that you just looked at in the dlcpacks