How can I find and edit my external hard-drive's .fseventsd folder?
Files and folders starting with a '.' are invisible in Finder by default. You have to activate this option in order to view/delete them.  

Open Terminal and typ defaults write com.apple.finder AppleShowAllFiles -bool true, after that, restart your Finder.
To hide those files again, open Terminal and type defaults write com.apple.finder AppleShowAllFiles -bool false and restart Finder again. 

But remember, they are hidden for a reason, so don't mess around with this kind of folders unless you are absolutely certain of your actions...