If you truely are trying to "BACKUP" SSIS packages, then you should only backup the following two things:
- Backup SSISDB database just like any other database
- Backup the Master encryption key for SSISDB.(Also keep save the password for SSISDB Master Encryption Key somewhere safe, you do need this while restoring SSIS Catalog)
You will need these two things to restore the SSISDB database.
Alternatively, if you are just trying to download SSIS packages from SSISDB, that have been already deployed, then you need to open Visual Studio, and create an SSIS project using template "Integration Services Import Project Wizard", this will allow you to connect to a server and download SSIS projects from SSISDB.