Make sure the keyframes are set in your FBX import file. In the import dialog, check "Split Animations" and set the keyframes to the beginning and end in accordance with your C4D file.
Or, you can do it with script:
function Start(){
animation.AddClip(animation.clip, "Walk", 0, 48, false);
animation.playAutomatically = false;
animation.wrapMode = WrapMode.Loop;
}
function Update () {
if (Input.GetKeyUp ("1")) {
animation.Play("Walk");
}
}