I downloaded latest from GitHub - hippojay/plugin.video.plexbmc: PleXBMC for KODI
This now works perfectly for me on Alpha 4 build, however for true integration I had to change to Amber skin
GitHub - pecinko/skin.amber: Amber skin for MRMC
Would be great if one day OSMC skin allowed integration but I know this is quite a limited request as most other Kodi skins do not allow this level of integration. Now using a Pi2 my boot time has gone from 76s to under 30s including loading of my plex files on amber skin.