I can double-click the file and a browser preview opens that looks like excel.... but it is basically just a browser preview feature and it is read-only.
The file came from a Windows machine.
This response makes a couple of assumptions:
You should be able to use Apache OpenOffice CALC to edit files in the Microsoft Excel file format of .xlsx as per the OpenOffice website:
You may download OpenOffice from this site: http://www.openoffice.org/download/
Alternatively, you can always use the MAC version of Microsoft Office 2011 available for purchase here: http://www.microsoft.com/mac/buy
If you have opened it in Excel but you are unable to select or change it, the sheet may be protected. If the sheet has been protected using the Tools > Protection > Protect Sheet option then a lock symbol will appear next to the sheet name in Excel 2011.
Sheet protection can be removed by using the Tools > Protection > Unprotect Sheet command.