Click Data Management > Data Loader
If you ever need a refresher and you don't want to search for this article, the Data Loader includes a step-by-step walk-through. Access it by clicking on "Start Tour" in the right corner.
The most important item to remember about the Data Loader is the requirement for the .CSV file to have the word "phone_number" (without the quotations) in cell A1 and the numbers to dial in cells A2 - A###.
Click on Upload files
Click on Browse to locate your .CSV file
Monitor the progress in the Data Loader Queue and confirm the successful upload by clicking on Uploaded Files
Your Dashboard will provide you with several useful pieces of information: