It's really quite simple:
1) Head over to the "Reports" menu and select "Reports by channel" (on the left-sided menu).
2) A page will come up with a stats summary for all email campaigns you sent out. If you'd rather view all reports for another mailing channel (eg. SMS), just hit "Email" on the upper right corner of the page and choose your desired channel.
3) Click the "Report" button (to the left of each report) and you'll be taken to the full campaign report. Then simply hit the "Export" button (top right) to download an Excel file with the report data. Done! :)