In the blender game engine, how do I make an animal change it's condition from walking to attacking when it sees the player?
You need to use states. Basically this will be :
Near property Player > And > State set state 2
It will look like this.
Now on state two, you need an Always sensor rigged up to a Steering acctuator with the player being the target object.
To avoid being stuck in state 2 forever, make something to switch back when the player isn't nearby anymore.