The best way to find files is through Terminal. You don't need to download or install any extra applications.
Simply issue the command:
sudo find / -name rt.jar
It will ask your for your password; enter it. Find will go through all of your directories and when it finds any instances of "rt.jar" it will output the directory to your screen. You can then navigate to that directory via finder or through the terminal via the "cd" command
you can delete it if you like with the "rm" command
If you want to see files in Finder just issue these commands in Terminal:
defaults write com.apple.finder AppleShowAllFiles TRUE; killall Finder
After that, you will be able to search and view all of the hidden files.
If you want to go back to the default view just change the "TRUE" to "FALSE"
defaults write com.apple.finder AppleShowAllFiles FALSE;killall Finder