This folder may be hidden by default in the Windows file system. If you cannot see it, go to "View" in File Explorer and check "Hidden Items" to display hidden folders and files.
The default library folder location for Proteus 8 Professional is: mq135 gas sensor library for proteus download upd work
If you continue to face issues, consider joining the or r/Proteus on Reddit – many users share verified working versions of the MQ135 library. This folder may be hidden by default in