the software center should still be there, just click on the dash button, usually on the top of the bar, and type in software, it should show up. Then you can right click it and add it back to your bar. Also instead of using software center to download steam, just go here : http://store.steampowered.com/ and click install steam at the top right, you can then download a newer version as a deb file. Once that is done just go to your downloads folder and right click the deb file, then open with ubuntu software center.