If you ever want to operate the account/s you have in the MyNxt Wallet
using the full Nxt Client, you have two options: you can install the Full UI plugin in MyNxt Wallet, which will give you a full running version of the client in your browser session, or you can export your MyNxt Wallet passphrase so you can use it in a local version of the core Nxt Client in your computer. If you want to do this, here's how.
Remember that MyNxt does not have the secret passphrase for the Nxt account you access using the Online Wallet, for obvious security reasons. When you operate in the MyNxt Wallet, your transactions are signed locally in your browser, and the secret passphrase required to send transactions to the network using that Nxt account never leaves your computer.
This secret passphrase (the one generated by the Nxt Client or yourself when you create a new Nxt account using the full Nxt Client) is calculated by the MyNxt software using your MyNxt credentials, most particularly your master password. This master password is not known by the MyNxt servers either (only a hash of this password is stored) and this is why it is VITAL that you keep your MyNxt master password safe. As you saw in the warning when creating your MyNxt Wallet, if you lose your master password you'll lose access to your account - the same happens with the secret passphrase for accounts created with the Nxt Client.
So, to open your MyNxt account with the Nxt Client:
1.- Log In to your MyNxt Wallet and have your master password handy.
2.- Go to Settings > Security
3.- Click on "Download Wallet Backup", and enter your master password. Note the reminder and click "Download"
4.- You need the MyNxt Wallet Viewer mentioned in the reminder above. You can grab it here
5.- Open the wallet.json.aes file you downloaded with the Wallet Viewer (you'll need to enter again your MyNxt master password). Then you'll see a table with all the accounts found in your MyNxt Wallet, each with the numeric Nxt account ID and the password for that account. Keep this password safe!
6.- Use this password to access the corresponding Nxt account in your local Nxt Client.