E-Maculation

I am following this websites guide to get MacOS System 7 working on Basilisk II, but in order to install the Installation files I need Stuffit Expander. I have downloaded it, but it won't open. It keeps saying

The Document could not be opened because the application that created it could not be found

. I don't know if this is a noob question, but please help me. And if I have to download something, please give me the link.

You do not necessarily need Stuffit Expander
Which is your host system?
Do you follow the setup guide in our wiki?

And in answer to your question: The Macintosh filesystem stores data differently than other filesystems. So if you download software to a Windows machine and then copy it onto a Mac image, you'll likely lose some parts of the file. This could be the bit that tells what application a document is associated with, or, in the case of applications themselves, you'll lose the executable part of the file.

This puts Stuffit Expander in a chicken/egg situation, as most files are compressed with it to get around this limitation of Windows, but you can't compress it with itself and then use it to expand itself at the other end.

One solution to this is outlined here: https://www.emaculation.com/doku.php/68k_software

Another is to mount a disk image with a copy of Expander on it, available here: http://macintoshgarden.org/apps/stuffit-expander-55 (select the .img one for download).