Do this:
Start Playonline Viewer as usual.
Choose cancel or anything you need to reach the main screen for POL, where you see the following buttons:
Member List, Information, Guest Login, Add Member, Install, Check Files, Join PlayOnline, Network Settings, Keyboard Settings, Quick Manuals, Exit Viewer.
At that screen choose "Member List", then click your account name. It will show an screen with four options, one being "Settings". Select it.
In the Settings screen choose the options so they look like this:
Member Name: <Whatever you want here as this is just cosmetic>
PlayOnline ID: <That code which is four letters and four numbers>
Set Password: (Select SAVE here to have it be stored)
PlayOnline Password: Password for POL, not SE account.
MEMBER password: <this is a local only password to prevent people using your PS2 or PC from logging into your account case you have the main password saved>
Square Enix ID: This is your user name for the SE account. Input it in the same way as you would if accessing SE account website.
One time password: Set it as "USE" if you own an active one time password generator and it's already registered to your account.