Are you browsing to the correct directory with your world? Like do you see it specifically listed there? I'm not at my MC machine, not sure specifically what file you have to choose, but try looking for level.dat under your world's directory (tho level.dat might just be for NBT editing). Launcher you use doesn't matter, it just a directory structure you have to navigate to, then find your save.
And you have the latest MCEdit right? I just downloaded the legacy mcedit 1.x (left hand side) from
www.mcedit.net the other day and it worked on my dw 1.7.10 world just fine.