The ISS is on a different freq than the main APRS network (North America is 144.39, the ISS is on 145.825).
You'll have to know when the ISS will be over your head. Google search "ISS track" to find any of a number of sites which will do this for you.
You may have to update your PATH. The information on the web is confusing and conflicting. I only ever connected once, over a year ago, and I don't recall if I changed my path or not. If you Google search "ISS APRS", you will find several pages with different paths that you can try (that's what I mean about it being conflicting).
If the callsign hasn't changed since I made my beacon, you will be looking for RS0ISS-4. Good luck!