Use Avidemux to re-pack the audio/video streams within the MKV file into the MP4 container - this should work as long the video is MPEG-4 AVC/H.264 and the audio is AAC, AC3, or MP3. I'm suspect that Premiere Elements has no issue about importing MP4 files.
Another approach is to demux the MKV into elementary streams and then import the audio and video streams. There are various programs that can do this - the most effective one for MKV files is mkvtoolnix with MKVExtractGUI2 (a separate GUI program that is dropped into the mkvtoolnix application folder).
