Are you certain that you've included "will_paginate" in your Gemfile, run bundle install
and restarted your server? If not, can you include the whole stacktrace?
Also, if you want to, you can restructure to Rails 4 style scoping like this:
def index
@users = user_scope.paginate(page: params[:page], per_page: 10)
end
private
def user_scope
current_user ? User.where.not(id: current_user.id) : User.all
end