You will need to also be verified, have an account for 30 days, etc at Dwolla.
You pick the "funding options", add funds, "Dwolla" in the drop-down menu, and then you will get text like:
You can send funds to:
Account number: 812-649-1010 (click here to send funds directly)
Account name: Mutum Sigillum LLC
You must include the following message: XXXXXXXX
NOTE: Your deposit may be delayed to up to 24 hours after your Dwolla transaction clears until Dwolla allows us to make this automated and due to more stringent AML requirements, all deposits are subject to a verification process.
There is a US$0.25 fee that you can either choose to pay yourself when sending funds, or we will deduct it from the received amount here.
You follow those instructions: log into Dwolla, and send funds to MtGox's Dwolla account number with a message including your code.