You need to download and run the Unlock.apk from Asus' website otherwise you won't be able to flash a custom recovery.
I've heard from other people that it only works on Jelly Bean so you might need to install Jelly Bean and then install and unlock your device.
Primary Phone: Asus Padfone 2 32 Gb + Padfone Station
ROM: My Custom ROM
MODS: Animation Framework Mod, Circular Battery Mod
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