I can't believe you don't have tar installed. But one of your earlier questions suggests to me that you may be trying the second line "extract gzipped foo.tar.gz" as a command, when Richard's response was simply an explanation of the first line. That is he said try
"tar -xvvzf foo.tar.gz"
Sorry, my earlier email was confusing. I meant, try the following command, replacing "foo.tar.gz" with the actual filename you're trying to decompress:
tar -xvvzf foo.tar.gz
If that says that the command isn't found, then try this which will install tar (I'm assuming you're using Ubuntu):
sudo apt-get install tar
If it says that it can't find the file (i.e. it can't find foo.tar.gz) then perhaps you're in the wrong directory. Do you know how to list the contents of directories and change to another directory?
If you still have the error, then email the exact error and I can help you troubleshoot further.
Richard