Forget about the barlow for now. Bung your 10mm in and go to the general area then do slow sweeps in the region. Neptune will stand out as blue and disk like or even as non-starlike. Your C8 will pick it up. Pluto on the other hand is a photographic job over a few nights. Your C8 will be straining and Pluto these days is wandering through rich fields of starry dots that look just like it. There's no distinction between Pluto and other mag 14ish stars except movement over a few nights. Visually, even with an accurate chart you will go spare trying to figure out which one is your target.
Richard