Apple ball speakers connect to MacBookPro?
You need a USB DAC (Digital Audio Converter).

I personally use a Behringer UCA202 to connect my iMac to my Bose Wave Radio (line input).  




What's really nice about it is that it's powered off the USB port meaning it doesn't need an external power adapter.  It also has both left and right inputs and outputs as well as a 3.5mm headphone connection.    For best quality, you will want to use the RCA output.  You will just need an RCA to 3.5mm adapter (second picture) to connect to your speakers.