Excel only has 256 columns up to version 2003. The compatibility pack won't change that, it only gives the functionality to be able to open .xlsx, but any data outside the previous version's limits will be dropped.
Option 1: split the file in multiple parts in Excel 2007 or 2010, each with maximum 256 columns. Also remember that the pre-2007 versions only handle maximum 65536 rows...
Option 2: upgrade your Excel version to at least 2007