Airport Express Not Found

I am trying to set up my second airport to boost the signal, and use as wireless speakers. i have an airport extreme which works fine.. i originally had a regular airport express,but then upgraded to the extreme, though when plugging in the old one my computer does not find it. it sees the extreme but not the old airport. how do i get it to find it. i am pretty sure all software is up to date.. even then how do i update the software for something my computer cant find?