If OVP fixed you're problem, then you're good to go, but I'm not here to stop you from using KMPlayer. I've personally had better luck with KMPlayer. I guess it's all on a "per video" basis so your mileage may vary.

For the install, you can absolutely use a terminal method but I personally preferred not to enable Dev Mode on my N9. It's an easy option if you like to do it that way, but I just feel safer(and lazier) using trusted deb packages. I'd say the simpler way is this:

1) Allow installations from non-Store sources on device.
2) Download the MPlayer and KMPlayer packages from here. You'll probably want to opt for the most up to date packages ("kmplayer_0.10.9.910-1_armel.deb" and "mplayer_1.0svn20120427-1_armel.deb")
3) Select those packages from the transfers list and install both individually. If you're uber paranoid, feel free to restart your device after each install. I got into a habit of it from the N900 days. It's a time waster, but it doesn't do any harm, it can only help.

That's really it! Let me know if anything wasn't clear. Best of luck!