An excel document very likely won't always open if you link to it. I'd recommend creating a PDF from the file, or compressing the file into a .zip format. Either option would be more reliable for accessing downloading and in the case of the PDF, could likely open in the user's broswer.
If you need the user to add data to the file, you could also use Acrobat Pro to turn the file into a form, so you maintain some form of interactivity.
Regardless, you will need to know the path the file, and ideally use Dreamweaver to link to the target document.
For any link you use the anchor tag. <a href="[file name]">[linking object, either text or an image]</a>. It doesn't matter what type of file you're linking to, although - as Jim points out - browsers don't normally render spreadsheets so your visitors will need to have Excel or some other program that can open it. If you have corporate customers, that's probably fine. If it's the general public, maybe not.
If the file you're linking to is in the same folder, you just need the filename. If the file you're linking to is in a subfolder, you put the subfolder name, a slash, then the file name. If the file is in the folder above, you type "../" before the file name. If you need to go up the folder tree and down another branch, you use "../" for each upward movement, then /[folder name] for each step back down. Hmm...try the video here for a visual: http://www.pepmint.com/tutorial/link_a.html