I don't know if there's a readme in the download you made, but anyway I find this very precise note by Bazza:
NOTE ON INSTALLATION OF THE LOTUS 49: You need a original kunos Lotus 49 install file:
1. Extract folders content and texture to your AC install: ...\SteamLibrary\SteamApps\common\assettocorsa\ 2. Goto …\SteamLibrary\SteamApps\common\assettocorsa\content\cars\lotus_49
3. In this folder: Copy the lotus_49.kn5 file
4. Within \cars folder, goto \ f1_classic_lotus_49
5. Paste lotus_49.kn5 here!