Use kingo but make sure USB debugging is checked AND UNKNOWN SOURCES IS CHECK no one reads exactly how your suppose to do it lol guerentee that's your issue as well unknown sources check it
Sent From My Galaxy Mega GT-I9205 With Xda Premium 4
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download Odin, download Phil recovery or cwm recovery, download Supersuer apk and flash both with odin. use Odin to flash to custom recovery to your Phone and then flash the superuser apk through the custom recovery