First download the file, and you should get a DigitalLogicSim(version).zip file. (Assume "version" is your OS.)
A zip file is a file that usually has multiple files within it (and is useful for reducing size of the files.)
WINDOWS:
To extract the files from the .zip, you need to right-click the file in your file browser, then click "Extract All".
For other versions of Windows, you might have to install a tool, (such as 7-Zip or WinRAR), to extract the file.
LINUX:
If you are on Linux, either use commands in the Terminal, (like this) or if you are on a more user friendly version of Linux, (such as Linux Mint) simply right-click the file in the File Browser and click "Extract Here".
Hope this helps.