This problem keep cropping up from time to time. Basically Apple uses a 5.1 digital optical connection and your speakers use 5.1 analog connections. You either need to get 5.1 speakers with digital optical in, or an adapter.
Adapters that decode optical 5.1 to analog 5.1 are very rare these days as Griffin stopped making them years ago. Only way to get one now is used on Ebay or some such.
There were also OS X-compatible USB sound cards that output to 5.1 analog, but these are also "out of print" and extremely rare. They may also not work on newer versions of OS X.
The easiest path here is to get a set of 5.1 speakers with digital optical input and recover part of the cost by selling your 5.1 analog speakers.