Hey there!
Try deleting all the files you installed and then downloading them again and installing them into the folder. Also, make sure you are using the latest version of the game and the modification.
I recommend carefully reading the installation instructions for this modification on the page: https://truckersmp.com/kb/84
Here's what it says on the instruction page:
"Not appearing in the Mod Manager"
Make sure that you have installed them in the correct directory, rather than in the standard American Truck Simulator or Euro Truck Simulator 2 folder, it should be in the ATSMP or ETS2MP folder respectively. Just like with ProMods, the Winter Mod files will not appear in your Mod Manager when you launch TruckersMP, this is normal. If the Winter Mod files are in the correct folder, they will be picked up by our launcher.