After the software has been installed and the drivers have been installed as well, connect the phone and wait for Windows to copy the necessary files so that your device will be recognizable on the OS.
Start the software, connect the phone again and open the application. Sometimes you'll probably be asked to enable MTP connection. After this, click the Music section on the software to upload or download the music to your phone.
Alternatively, access the phone's storage from the My Computer/Computer section in Windows and use Copy and Paste for your MP3 files. You can place them on Music folder on both places Internal Storage or External Storage as Android will recognize them as long as you use a compatible audio player.