Well.. I ended up reinstalling Java (the installer told me I had it installed already, as I thought). Then I downloaded it again and again as thought, it was something quite obvious that I overlooked. My file was called "Metropolis.jar.zip"; by simply removing the .zip extension it works a-ok now. Thing is, when extracting the files like any other archive (for which I use 7zip, not that it really matters), it would give me these files: http://puu.sh/8wIJr.png