Two things.
(1) It helps to tell them what the format of the trigger is !<rolls>d<dice sides>+<additional value>
(2) If your not going to show the dice rolls why do em?
Code:
on $*:text:/^!(\d+)d(\d+)(\+(\d+))?$/g:#: {
  if (($regml(1) isnum 1-200) &amp;&amp; ($regml(2) isnum 2-1000)) { msg $chan $nick $+ , You rolled $calc($rand($regml(1),$calc($regml(1) * $regml(2))) + $regml(3)) in total. }
  else { .notice $nick Format !&lt;rolls&gt;d&lt;dice sides&gt;+&lt;additional value&gt; The number of die that are rolled must be within 1-200 &amp; The number of sides on the die must be within 2-1000 . }
}