Transferring bitcoin from old bitcoin core wallet to electrum
I have a 7 year old wallet.dat file

I guess that predates HD wallets. Which I think means you'll need to sweep all private keys which have money (UTXOs) currently associated with them.
For an HD wallet I believe you can probably sweep the Extended Private Masterkey shown at  the top of the dumpwallet output.

I'm stuck at the step dumpprivkey, because I don't know what &lt;your bitcoin address&gt; means.

listunspent should help by giving you a list of addresses which actually have money associated with them.