launch your bitcoin client as usual and wait for it to load the blockchain and start up
click on 'help' in the menu bar (top right)
click on 'debug window'
select the 'console' tab
type:
type:
this will return the private key, you can copy it now; ensure you clear your clipboard/history afterwards
type:

If you never encrypted your wallet, then you don't need to do the "walletpassphrase" command. That's only needed to unlock the wallet if it is protected with passphrase encryption.
As far as the public key for dumpprivkey, that is asking what Bitcoin address you want the private key for.
To get the list of Bitcoin addresses in your wallet, you can use:
or