In Finder, press Command + Shift + Period
In Finder, open the Go menu and choose Go to Folder
Type the hidden folder path, such as /Users/yourname/.foldername, and press Return
In Terminal, run ls -la to list hidden files and folders
In Terminal, run defaults write com.apple.finder AppleShowAllFiles true
In Terminal, run killall Finder to restart Finder and show hidden folders
To hide folders again, run defaults write com.apple.finder AppleShowAllFiles false
To hide folders again, run killall Finder