slimerancher.exe would be in your SlimeRancher folder in the SteamApps secton of your Steam directory.
If you don't own SlimeRancher there is no reason to buy that mod.
Also, this is the itch.io forums ^_^ You're posting in the wrong place. You should email questions to the developer of that mod, or ask your question in their discord (link provided on the Better Build page).