Open the Kits configuration pane via "Qt Creator -> Preferences", then add a Qt Version in the Qt Versions tab. Switch to the "Kits" tab and create a new kit. Click the "Qt Versions" dropdown and notice the new Qt version is not available. Close the preferences, open them again, notice that the new Qt version is now available.
This may be specific to Mac OS. On Mac OS we don't have an "apply" button in the Qt versions tab. However, the "apply" button is a crutch in the first place. Any Qt versions defined in the Qt versions tab should automatically be available in the kits tab.