To make it clear for you to see, here is another example:

on *:TEXT:!register:#:{
  if (!%coke. [ $+ [ $nick ] ]) {
    set %coke. $+ $nick 1
  }
  else {
    msg $chan $nick you already have an account!
  }
}

The $2 is not necessary. Simply use the !register command. When you set a dynamic variable, you don't need to include the [ ] evaluation bracket.