MXF files in Premiere Pro cs5 mac only playing/dispaying audio
you could also try an intermediate workflow, like transcoding the MXF files into an appropriate ProRes container. This most likely would be 422Proxy. 

The Canon C300 codec is a variation of XDCAM (source: http://www.premiumbeat.com/blog/editing-with-canon-c300-footage-tips-for-when-it-all-goes-wrong/) I personally do not like to edit in Long-GOP or predictive frame codecs. So transcoding to an inter-frame codec like ProRes or DNX is always a safe bet.