Do I need to put my legal name in the activation pop up for Poser 11?  Or can I use any name like a "real" name?

I would suggest you use a name you can prove is yours or one you have on the payment method you purchased the software with. It is a legal agreement.

Else, some-where down the line, your attempt to buy an up-date, eg from P_12 to Poser'23, will go horribly, expensively wrong: 'No such name, no such number...'

( 'Accounts & Purchasing' are funny like that: I managed to crash the payment system of a 3D site because it said to enter my name exactly as on my debit card. A  'Bobby Tables' failure ensued, as their 'front end' could not parse the multiple dots of my initials. And a big UK cell-phone company did exactly the same, to their profound chagrin... )

Oh, I gave up on entering the special characters of both my first and last name as soon as I arrived to the US which was 22 years ago. Dots and  's and "s don't exist properly above any vowel here.

Please write down your serial number someplace safe too. And, that's not in a file in the same computer that will eventually crash. Print it out and put it someplace truly safe. Do use your your real name as it helps support track down your serial number if you didn't write it down.