Please go to Tools->Options->Build & Run->Kits and check that there is a Qt version set up in the kit you are using.
If not, then check the Qt versions creator detected in Tools->Options->Build & Run->Qt versions. Is the version you want to use there? If not: Add it here.
Then return to the kits page and set one of the kits to use that version. Double check the other information while there.
Now you should be able to build your project. You might need to close/reopen it though.
Initial setup seems to be a bit bumpy... but it works really well once you get over that initial hurdle:-( There are quite some improvements in the next update for this. Check the Qt 5 RC2 packages, the creator included there already contain most of the fixes if you are curious).