[Eikinskjaldi]

....and another variation that requires less work from you but a little more from the users:

<script type="text/javascript">
function dropDownDice(obj,obj2){
    var diceToRoll = document.getElementById(obj).value;
    var faces = document.getElementById(obj2).value;
    document.getElementById('{$editorid}_textarea').value += '[dice='+diceToRoll+']'+faces+'[/dice]';
}
</script>

<td><select id="{$editorid}_dicetoroll">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
</select></td>
<td><span style="color:black;"><b>D</b><span></td>
<td><select id="{$editorid}_dicefaces">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
</select></td>
<td><input type="button" onClick="dropDownDice('{$editorid}_dicetoroll','{$editorid}_dicefaces');" value="Add Dice" /></td>

same rules apply as above, Attaching a screenshot of this one since it is a little different.