This is an acknowledged problem of Linux Mint 12 KDE release. The current workaround is to use Muon as a replacement for Software Manager. See here: viewtopic.php?f=109&t=93496
Clem has confirmed the issue, as also described in above link, and the development team will be working on fixing this as part of the Linux Mint 13 development cycle.
For now, you can use Muon as a workaround to this problem. Please have confidence that Clem is aware of the issue and that the development team will be working on fixing this.