why can't I open up xls files?

xls files are Excel Spredsheet files. Do you have Excel installed so you can open the files in Excel?

You should be able to open XLS files with either Excel or Numbers.

Allan

...and if you want an application that is free, you might consider OpenOffice for Mac.

http://porting.openoffice.org/mac/download/index.html

Another possibility... If you have an older version of Excel, and the file you are trying to open is a new version with an .xlsx extenstion, then you need a converter to open it. (Sorry, I don't remember where to get it, because it didn't work for the first year that I tried it before I upgraded to Office 2011 at work.) Weird, since I'm able to open them on my iPhone, but not on my iMac. I've asked friends to save as an older version; some know how to use "save as".

It can also happen cross-platform. If your file is from someone using a new version on a PC, and you don't have the equivalent version on the Mac, it might not open. On the PC, the extension doesn't show, so they may not know what version it is. But when they pen Excel, it will say as it opens.

If you do not have Microsoft Excel (you can look in you Applications folder) then you will need another program (Numbers, part of iWork, and Open Office are possible applications people have already suggested). I would also like to suggest LibreOffice (http://www.libreoffice.org/)