The speaker output connections on this stereo system unit are called RCA...named from the inventing company. Computer speakers use what is called a "mini-plug" and as you've found out, the two are not the same.

I would suggest checking at your local Goodwill or Salvation Army store for an inexpensive pair of speakers with the proper RCA connection you need. Second choice is the local Radio Shack store...tell them you need speakers with RCA type connections and you will also need two RCA to RCA speaker wires to make the connection needed.