Mailing tool not finding Excel spreadsheet

Hi,

I am trying to create multiple docs at once using mailing tool, but when I try to merge its trigger a following error "This file needs to be opened by the Excel Workbook text converter, which may pose a security risk if the file you are opening is a malicious file.  Choose Yes to open this file only if you are sure it is from a trusted source." I have check if I have any pending updates and I am ok as change Excel format to xls, nothing work. need help.