How to open excel .XLT file

Ask on the Microsoft Mac forums, since it's their software you're working with

http://answers.microsoft.com/en-us/mac

.xlt is an old Excel extansion for Excel templates (Excel 2004 for Mac)

https://support.office.com/en-nz/article/Sharing-documents-with-other-versions-o f-Office-fdc95a18-f1fe-4049-b85c-42e2143f9243?CTT=5&origin=HA102927257&Correlati onId=b341cc24-ab88-4d53-b07d-bc99eeda9ca4&ui=en-US&rs=en-NZ&ad=NZ

Try to customize Excel for Mac first, here is the guide how to for Excel 2011 for Mac:

https://support.office.com/en-us/article/Customize-how-Excel-opens-workbooks-and -sheets-8f3c2396-3883-4e3e-a668-8254aeff6a0f

After that you could try to open the .xlt file and save as .xltx file.