I'm missing something in your question. Is the problem you cannot download and install Desktop Manager?
Did you get a CD with the BlackBerry when you acquired it? If so, you will be able to install desktop manager on your PC from the disc.
After that, take a look at Synchronize your music from your computer to your device using BlackBerry Media Sync - User Guide - BlackBerry Curve 9300 Smartphone - 5.0
Also, once you have the drivers installed and you are able to connect to the BlackBerry, you can just enable and use mass storage mode to drag and drop files to the media card, if you desire.