How Qt Creator auto-detect qmake location?

I tried to install V-Play [v-play.net] (A Qt plugin for game dev come with a custom Qt Creator)and uninstall it. Now when I reinstall Qt Creator 2.5.2 (base on Qt 4.8 in Ubuntu Software Center),. Go to Tool >> Option >> Build and Run, Qt version Auto-detect the path of the VPlay installation (but I have removed it). I must add a manual version(at usr/bin/qmake). My question is how to Qt Creator detect the path? And what I can do to the auto-detect path is what it must be? Thanks!