Probably an error with extraction and/or location. Some Zip programs mangle file names, and RPGMaker is particular about its filenames: if they get messed up, things break. RPGMaker also HATES running in deep directory structures for some reason. For the sake of testing, I'd extract the zip with the default built-in functionality of Windows, then make sure the folder is on the root of your drive (though in theory it should be able to handle being a few directories deep).
Also this is a minor thing, but just to be clear, there is no "Version 0.5.1-.6" It would be 0.5.1, 0.5.2 etc. If you downloaded the most recent, then it should be 0.5.6.