The Simply Investing Spreadsheet is in a Microsoft Excel file format.
There are two options for you if you don't have Microsoft Excel:
Install OpenOffice, it is free and it will open Microsoft Excel files. You can download OpenOffice from here: https://www.openoffice.org/
Google Sheets is free to use (but you have to create a Google Account) and it can open Microsoft Excel files. You will need to upload the Excel file to your Google Drive account and then use Google Sheets to open the Excel file. Instructions are located here: http://www.documentsnap.com/convert-excel-to-google-sheets/