Been doing some initials tests on TmK and am really pleased with everything.
I used your example TileMaps and stored them in the root of the project with the tilesets in a separate subfolder, everything worked fine.
I then brought some TileMaps that I have been working on for my next project and placed them in a subfolder "TileMaps" with the TileSets in a subfolder under that. My line of code is:
let myTileMap = TKMapNode(contentsOfFile: "TileMaps/level001.tmx", renderSize: CGSizeZero)
I then get this error:
2015-09-06 23:29:37.988 TMKTest[1363:522117] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSURL initFileURLWithPath:]: nil string parameter' *** First throw call stack: (0x185731a94 0x19a17bf80 0x1857319dc 0x1865daa94 0x1865daa00 0x10003db3c 0x10003da28 0x10003d5a4 0x100060b50 0x100045f9c 0x100045e0c 0x10006417c 0x100063a50 0x100063b0c 0x18aa95128 0x18aab0240 0x100064abc 0x100064b78 0x18ac42aa0 0x18ac427b8 0x18ac492d8 0x18ac467bc 0x18acba86c 0x18aee00c0 0x18aee413c 0x18aee12e8 0x18f3877ec 0x18f387b6c 0x1856e90dc 0x1856e8b70 0x1856e6870 0x1856158a0 0x18acaf1ec 0x18acaa0d4 0x1000661a0 0x19a9ca8b8) libc++abi.dylib: terminating with uncaught exception of type NSException