We aren't going to be able to help you much as you didn't give us any info such as symptoms, error messages, etc to go on. I have the Sony 600 and Calibre works great for me.
Offhand, the only thing I can think of is to make sure you are closing down Sony Reader Library before you bring up Calibre. Calibre cannot transfer stuff if Sony Reader Library is up. This is mentioned in the Calibre instructions.
Whether or not Sony or Calibre are up you should see USB drive letters on your computer when you plug your 300 into a usb on your computer. Your 300 mounts Removable Storage Devices (usb drives), one of which is where the books download from Sony through and transfer through from Calibre to your physical book reader device. In my attachment you can see that my 600 mounted 4 drives: I, J, L and Installer for Reader. You will probably only have 2 drives, as my 600 mounts a drive for each of the 2 card media slots. If you don't see drives then your problem is bigger than Calibre. If you do, then make sure Sony Reader Library is shut down and only then bring up Calibre.