I read that to create a Death Knight all you have to have is a lvl 55 or higher character.  I have a lvl 56 warlock but when I go the the create a character portion, it shows the D.K. symbol, but I can't select it...what am I missing???

1) Are you trying to create the DK on the same realm as your lock?

2) Do you have 10 charaters already? (since 10 is the max)

If your answers are 1) Yes, 2) No, then contact a GM

You need Wotlk unlocked.