The Lubuntu software centre in Lubuntu 13.04 Beta is not yet populated. In other versions of Lubuntu, the software centre works fine, but there are more programs available in the Ubuntu software centre than in the Lubuntu software centre, so you might want to add the Ubuntu software centre anyway.
To add the Ubuntu software centre, open a terminal (Lubuntu menu \> Accessories \> LXTerminal) and type sudo apt-get install software-center. Make sure that you are spelling software-centre in the American way in stead of the British spelling (software-center, not software-centre). Type your password when prompted and press the return key. Then type y and press the return key when it asks wether on not you would like to continue. And wait untill the process is finished and you get a normal prompt. Then close the terminal window and if you want to check out the Ubuntu software centre, select it from Lubuntu menu \> System tools \> Ubuntu software center.