I changed the starter character to a customized r15 block dummy made with the rig builder plugin. I changed the idle animation successfully, yet I am entirely unable to change the walk animation? I've tested with the animation loop on, off, different priority levels, yet it still uses the default roblox r15 walk!!
In Roblox, when you enter the game, there's a local script called "Animate". There are String Values that are called different actions such as "Walk" or "Climb". Then there's a child of those that is the Animation Object. You have to customize the ID to the custom animation you exported (presumably you did) and then it should work.