Usually, the only program that can work with .numbers spreadsheets is Numbers (previously iWorks Numbers) from Apple, which is available on OS X platform. Windows users have notoriously problems with accessing this spreadsheet file type and usually prefer Excel files (.xls, .xlsx) or some open productivity suite alternatives.
Your best shot for conversion of numbers to xls is to ask one of your friends that owns Mac to do the conversion for you, because Numbers app is free download since 2012. Also latest versions of LibreOffice Calc supposedly can work with .numbers files, but this may only be the case for some older variants of the format.
The last option would be to use some of the online conversion services like ZAMZAR, but the results may not always be what you expected, especially in case of complex spreadsheets.
Updated: August 1, 2016