Since I don't know what you "get" when you download it from file planet you might just contact support. Sorry I can't be of help.
It was my thought that you would have a zip file or some sort of file that is an archive of all the C4 files. Just open it and drag it to a folder on your hard drive. If this is not the case contact support.
Don't know if you get the same stuff from fileplanet as you do from the L2 site, but winrar has an issue with trying to unzip the files. So if you're trying to use winrar stop and use the windows unzipping utility. Right click on the zip -> open with -> Compressed(zipped) folders. This will then open up a window showing all the files. Drag and drop them to your desktop or something then click on the setup.exe