这里这个down的用法是非常随意非常口语化的,这里指的是愿意或同意做某件事,you are down for somthing可以理解为you want to do something或you committed yourself to do something或you are okay with something.
举个例子：
A：we are going to the movies,are you down?
B：yeah i'm down.
这样就容易理解了,A问B要不要一起去看电影,B表示愿意一起去.
原文要翻译的话,就是你愿意为我做这件事儿么?具体还要联系上下文来看.