I've not used this, (or any frequency app actually), but you could try the Continental app mentioned here:
http://www.ukmonster.co.uk/monster/s...ad.php?t=56189
Make sure you pluck at the right points, (see here
http://www.ukmonster.co.uk/monster/s...t=58330&page=2). And most apps need a second or so to register the frequency so pluck once and let the frequency settle before trying again (rather than constantly strumming it).
Hope that helps.