Hmm...
Some websites don't know how to handle disk image downloads, and your disk image that you're trying to download gets encoded with ASCII instead of MIME/application-type. So you end up seeing a bunch of gibberish text in a browser window.
In your case, you're seeing this in Excel.
Stuffit Expander is a free download, just google it. It's a decompressor for archived files, like .zip. Lots of mac apps come in Stuffit .stx files. Instead of clicking directly on the aim installer file, right click and specify to download to desktop. Then double click it, but download stuffit expander first to 'unzip' it.
EDIT: It's not the website you're downloading it from in this case... it's that you don't have stuffit. So when you click the installer file, it's trying to open it in excel, and will end up displaying text gibberish.