Converting AS2 to AS3 and running into errors

In addition to the currentLabel property, the AS3 MovieClip class already keeps an Array of all timeline labels for you in the currentLabels property:
http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/display/MovieClip.html#currentLabels

Each item in the array is of type flash.display.FrameLabel, which has properties frame:int and name:String.

The code could be updated and reduced to this:

import flash.display.FrameLabel;
function NEXT(event:MouseEvent):void
{
    for (var i:int = 0; i < currentLabels.length; i++) //get index of current label by matching current label name
        if ((currentLabels as FrameLabel).name == currentLabel)
            break;
    if (i < currentLabels.length - 1)
        gotoAndPlay((currentLabels[i+1] as FrameLabel).frame)
    else //whatever you want .. like jump back to the the beginning
        gotoAndPlay((currentLabels[0] as FrameLabel).frame)
}
next_btn.addEventListener(MouseEvent.CLICK, NEXT);
function PREV(event:MouseEvent):void
{
    for (var i:int = 0; i < currentLabels.length; i++) //get index of current label by matching current label name
        if ((currentLabels as FrameLabel).name == currentLabel)
            break;
    if (i > 0)
        gotoAndPlay((currentLabels[i-1] as FrameLabel).frame)
    else //whatever you want .. like jump to the end.
        gotoAndPlay((currentLabels[currentLabels.length - 1] as FrameLabel).frame)
}
prev_btn.addEventListener(MouseEvent.CLICK, PREV);

Also worthy of mention is the difference between the currentLabel and currentFrameLabel properties, which are slightly different.  I've paraphrased/corrected the documentation to clarify it.

currentFrameLabel: The label, if one is present, on exactly the current frame.  If the current frame has no label, currentFrameLabel is null.

vs

currentLabel: The label, if one is present, on exactly the current frame OR, if one is not present on the current frame, the next earlier frame that happens to include a label.  currentLabel returns null only if neither the current nor any previous frames include a label.

More than likely, you want to use currentLabel to avoid null values when the playhead is on an unlabeled frame between your labeled frames.

You can probably just rename that variable to something else to solve the error issue.  AS3 has a currentLabel property for the MovieClip class, so anything with a timeline essentially has that proiperty.  IF you rename it to something else the error should go away.