If you are downloading mods from third-party sites rather than the Steam Workshop, follow these steps to install them correctly: Step 1: Locate Your Mod Folder
Follow creators on the SCS forum to get notifications on updates. Conclusion
Version 1.15 is very old (released ~2014–2015). Most mods for this version are no longer hosted on mainstream sites like the Steam Workshop (which requires newer game versions). You’ll likely need third-party archival sites.