In the NXT register alias dialog pop up, what is the DEADLINE text field for? What is the deadline text field for in NXT register alias?
The deadline field is the amount of time in minutes/blocks (1440, or one day being the default) for the transaction to complete. If it is not completed in that time period you receive a "refund" from the network and the alias will not be registered to you. 

A common mistake is to set the deadline to 10, or an even lower number reducing the odds of your transaction going through.