Why aren't my PPA's showing up in the Ubuntu Software Center?
If this is the same bug that's been around for as long as I can remember then you need to run sudo update-apt-xapian-index -f to get the Software Center to update your added repo's. On further research, there are bug reports for this issue not showing added repo's and also for continuing to display repo's that have been removed. Either way, running this code will work for both. As of 13.04 this issue still persists.
This issue can be partially resolved by installing something from the PPA via terminal first. Only then will you see the PPA appear in the Software Center.