Activity Monitor columns missing
You can reset your Activity Monitor settings by quitting Activity Monitor, and then  deleting the preferences file, and any preferences lock files, and then re-opening the app.
Open Terminal and paste in the following command:
rm ~/Library/Preferences/com.apple.ActivityMonitor.plist*

(Note the * at the end to delete com.apple.ActivityMonitor.plist.lock and com.apple.ActivityMonitor.plist.)
Hit enter, and reopen Activity monitor - the columns should now be present.