HT4724: Final Cut Pro X: Importing Sony XAVC, XDCAM, XDCAM HD, and XDCAM EX mediaLearn about Final Cut Pro X: Importing Sony XAVC, XDCAM, XDCAM HD, and XDCAM EX media
Maybe the article can help you: http://mts-to-aic-converter.com/import-sony-xdcam-media-to-fcp-x/
I use XDCAM Browser nearly every day with no problems at all.
What is your workflow?
It sounds as though you are trying to use the native files in FCP X - and this doesn't work.
With XDCAM Browser, you need to select your clips, then right-click and choose 'Register for Final Cut Pro' to re-wrap the files for editing in FCP X.