Hi..you can just use Kingo Root from this
thread which is really simple to use, but i see the link for the program is now working again in that post,
but you can get the program in my post also here as it will work on your model of the mega and use either the CWM or TWRP recovery from Silesh.Nair post
here and in his signature use the downloads link and once on the download site and then select Recoveries via Odin and then select Old and there you can choose either CWM or TWRP recovery to flash with odin (i use Odin 3.07) and flash the .tar file with Odin and check my post
here on how to use Odin, hope this helps you out and if you need more help just post.