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Sorry, I got stuck under 3.
3.- Click on "Download Wallet Backup", and enter your master password. Note the reminder and click "Download"
I have master password (and public key).

I did:
opened browser: google chrome
loged in to nxt online wallet
clicked browser setings>security
there is no "Download Wallet Backup" 
and here I'm stuck.

If I'm guessing right, the problem is you're looking for "Settings > Security" in your web browser, while the tutorial refers to the Settings option in your myNXT wallet dashboard. I've edited the post to make it explicitly clear.

6. NXT Desktop: where do I put in this password for my account?

Once you have decrypted the passphrase from your downloaded wallet backup, you have basically two ways of using the passphrase with your Nxt desktop client.

1) Just access to your account like you did, using the address, and then introduce your passphrase only when required by the client (for example, at the moment of signing/sending a transaction).
2) In the Nxt client login page, you can click on the key icon to log in using the passphrase instead of the Nxt address. This will open the account that belongs to that passphrase, which can also confirm that it was entered correctly (that the corresponding Nxt address matches).

Ok, now I see the confusion is different.

The instructions to download a wallet backup / export the account passphrase from the myNXT web wallet always refer to steps to follow in the myNXT web wallet, https://wallet.mynxt.info. Not to the official Nxt client, the one you can download from https://nxtplatform.org/download.

These steps before myNXT closes (and again, myNXT is a third party service and unrelated to the Nxt core development, so this is just a third party tool closing, nothing else) are one of the two options for myNXT users (see their FAQ) to either transfer tokens to another Nxt account managed with the official Nxt client, or obtain the passphrase for the Nxt account they managed with the myNXT web wallet.

All Nxt accounts are unlocked by a passphrase, equivalent to the private key in other platforms like Bitcoin or Ethereum. When you create an account using the Nxt Client, you are reminded to save the 12-word passphrase that the client generates. This passphrase is necessary to send any transactions from the account. But the myNXT web wallet, for design and security reasons, does not work directly with that passphrase, but using a separate "master password" to encrypt the user data, including the passphrase for their Nxt account, in the site database. So any user who wants to have the actual passphrase, to use that same account with the Nxt Client, needs to download it from myNXT and decrypt it, as the instructions say.

Since you say you already had generated a passphrase + account using the official Nxt Client, you have the two options in the FAQ linked above. You can transfer your assets + NXT from your myNXT web wallet to that account you already have the passphrase for, or you can download a wallet backup from myNXT (https://wallet.mynxt.info, settings > security from the dashboard) and then use the passphrase obtained to access the same account in the Nxt Client.

Dear VanBreuk,

My situation is that I created a NXT wallet by using official NXT client, then I missed one word when storing the passphrase (maybe the last one). I tried to use ardor.tools/typo/typo.html but it didn't work. How can I recover my passphrase because I have balance in this wallet? Thank you.

Hello,

since that is not a myNXT wallet issue but a general Nxt issue, please check this help topic in the Jelurida support desk. But the loss of a word in the standard Nxt passphrase sounds critical, specially if you're not sure of the position the word was in.