[lubuntu] md5sum does not work

I have just downloaded the lubuntu12.10 iso and tried to check it.

Neither of the instructions for doing this here:

https://help.ubuntu.com/community/HowToMD5SUM

are giving me any joy.

automatic method gives me: command not found

semi-automatic method: I really can't work out what I'm supposed to download here, or how...

Any help much appreciated :D