You can use the Canon XF Utility stand alone app (not the FCP plugin). It's a crappy app but it rewraps the media into a standalone MXF file (which can then be transcoded into ProRes or whatever you need).
Or you can use Adobe CS6 - Prelude and Adobe Media Encoder read the format natively and can rewrap it. You can download the 30 day trial to test it out.
Since Canon XF Plug-in can not help them, only the 1280*720 video accepted. It means you have to convert C300 MXF footages to Final Cut Pro compatible format ProRes 422 .mov with a top MXF Conveter for FCP. I used UFU Soft Mac MXF Converter, which can fast transcode SD MXF videos to Apple ProRes 422 for FCP on Mac OS X with best vido quality. and From this article,you can Learn More about Canon C300 MXF on FCP for editing.