you have to tell it you want to use a password, and it doesn't matter what user you run the command as, the user they're talking about is what you put after the -u option. I think you want to do this:
mysqladmin -u root -p creat phpbb2
then it will prompt for the password you made. I hope that works. I'm not too sure of the syntax. If that doesn't work try this:
mysql -u root -p
then type your password, then at the prompt type:
create database phpbb2;
then you can quit and it should have worked.

edit
you may have to use -h hexnet-server.