Activate the list of modules, Debug - Windows - Modules, to see which libraries are loaded, and if debugging symbols are loaded for these libraries. Please note that for this you need the debugger to be active in your process.
In debug mode, Qt libraries should be marked d. QtCored4.dll, for example. If you see QtCore4.dll in the list, you have contacted the wrong libraries. If he says QtCored4.dll, but in Symbol Status there is something other than the symbols loaded., Then this is the reason. You can manually load characters here by right-clicking on the dll and selecting Load Symbols. It requests a Pdb file after clicking on it.
You can add additional locations of pdb files if they have not been downloaded, but exist, from Tools - Options - Debugging (tree) - Symbols (tree). If you want the characters to be automatically loaded.
Should this give some error that the pdb file does not match the binary or something like that, you should rebuild your debug version of Qt if you don't have more pdb files. Use the Visual Studio command prompt and change to the Qt directory, run configure and nmake. It will take some time, maybe a few hours. There was some transition just to create a debug version, but I forgot where it goes. Please note that the assembly may fail, and after that you will really like the search on www.google.com. The best bet may be to get only the source package, I forgot where to get it, and it's not easy to find. If you know where to get it, please tell me. I vaguely remember that there may be some other way to create a debug version, but this is the only thing I know for sure.