DDE Issue with Mail Merge Using Word & Excel 2013

Is there a reason for using DDE other than the formatting issue? That is a severely limited connection type that restricts the mailmerge to using data on the first sheet in the workbook and starting on the first row of that sheet.

Your reference to "it will only show 6 columns of fields" suggests there are other issues with your data and/or your Office installation. Have you tried repairing the Office installation (via Programs & Features > Microsoft Office > Change in the Windows Control Panel)?

By default, Word (2002 & later) uses the OLE DB provider to get merge data from anywhere in an Excel workbook. Although DDE does preserve the source data format, Word's mailmerge field switches give you the ability to format the data returned by the OLE DB provider quite independently of how it appears in the data source. For a guide to using those switches see, for example:
http://www.msofficeforums.com/mail-merge/21803-mailmerge-tips-tricks.html

The only problem you're liable to encounter with the OLE DB provider is that it's designed to work with a single data type for each field. In Excel, however, cells in a column (field) can have different data types. Records that don’t conform to this convention are liable to not be handled correctly. The above link describes how those issues can be managed.