C:\WINDOWS\system32\mjextman.exe /play treepath=playlists\om
When I put this into the Run command for testing, it opens MC9 ok, but I get an warning box saying "Media Jukebox was unable to play any of the files in the playlist. Please make sure that the path int the Media Jukebox library points to the right location. Also double check to make sure that you are not unintentionally filtering these files." Click the OK button twice, and I'm back to MC9.
I haven't actually tried the command line options in MC9. I've been using MJ8 until Matt takes off his warning about getting more bug reports than could be fixed...
I have gotten that error, but it was due to some stupid errors I had made. First, make sure your back & forward slashes are correct (FWIW, your command above looks right to me in terms of this). Second, I don't know if case matters, but you might try "TREEPATH" and "Playlists". Also, make sure that your playlist isn't in a "Folder" (like "Mixes", etc.). The TREEPATH modifier needs the full path to the file, just like Windows. Finally, make sure you are pointing to the correct path for mjextman.exe. In MJ8, it's actually in the MJ install folder. I seem to recall that it had been moved for MC9, but that's something to check out.
Let us know how you fare...
Brad