Make sure you have Filezilla or an other FTP software installed in your machine.
When you chose to deploy your application via FTP at the application creation, the system creates a free FS Bucket with an ID matching your application's ID.
Find the FTP credentials in the configuration tab of this particular FS Bucket.
Just follow the instructions of your FTP Software to send code to Clever Cloud.
Refer to Quick Start - FTP deployment for more details.