Convert MKV to an iMovie-friendly format: MP4 or MOV with H.264 codecs, Apple ProRes, etc. I recommend.
Even if your file seems to be supported by iMovie because its file extension says so, its unsupported video codec may retard the work of MKV iMovie importing. Transcoding MKV files to H.264 is the recommended method.
I’ll show you how to manage the task with WonderFox HD Video Converter Factory Pro. It’s super easy and fast, check it out.