When I try, I'm told:
"The user name and password you entered do not match any accounts on record. Please make sure that you have correctly entered the first email address you used when you signed up for Nokia Messaging."

Problem: Some months ago, I was fooling around with this push email thing. If at the time I tried to create a Nokia account by inputting a user name and password, I don't remember what they are.
I wish Nokia would have some sort of recovery program . . .

I have also tried to start from scratch by hitting "Sign Up".
When I input my phone number, and select the N900, it tells me:
"Your mobile phone already has the latest Nokia Messaging software installed. To setup Nokia Messaging directly from your phone, go to Menu > Email > New > Follow on screen instructions."
When I input my phone number, and select the 5230 (my other handset), I'm told: "Invalid mobile phone number."

Looks like I can't win . . .