There are 2 different types of reports you can export to .csv, Campaigns Summary and Campaigns By Day. Both are exported form the Campaign Performance view. To access the Campaign Performance view, click on Campaigns on the left:
The Campaign Summary report will display campaign data for all given campaigns in your selected time-period. Choose a time-period by clicking on the Calendar icon, and then click Export and select Campaigns Summary:
The Campaign Summary report will download as a .csv for your convenience. This report essentially puts all the data you see in the graph and the table on the Campaign Performance view into an excel spreadsheet. It also contains a summary of all rows at the bottom of the report.
Campaigns By Day
The Campaigns By Day report will display campaign data as of today. Click Export and select Campaigns by Day.
This report will be emailed to you. This report will show you data for all of your campaigns as of today.