The x file is inside the rar file. When you have large files or a lot of files that add up to a large size, you use a compressor to compress the files into a new file, commonly .rar or .zip.
rar files are the current best compression type as far as I know, and require WinRar to uncompress the files inside. Just go to download.com, search winrar and download the link given, once you've downloaded it install it and you'll be all set. To uncompress the rar file with the model inside it, right click the file (it should have a stack of books beside it), and select Extract Here to extract the contents to the same folder as the rar file, or Extract to Folder if you want to extract the files to a different folder. Now that the files are extracted, you can do whatever you want with them.
For more info search rar files in google.