I have project in MFC that works fine in x32.
I used the configuration manager to change the project to x64, then I tried to rebuild the project.
the result was "unresolved external symbol" for many MFC function.
I guess that I miss something here in the Tools/Option toolbar with x64 platform but I don't know what.
P.S.
win32 console application works fine when I change the platfrom to x64
For our apps, it was a matter of changing to x64 using the Configuration Manager in VS 2005, recompiling, and fixing a few things here and there as prompted by warnings. It worked fine, although due to the size it took a while to get through everything.
In other words, what you did should have worked, assuming you did it properly. You have provided no details on how you changed the project, or what functions are "unresolved external symbol", or any details of configuration. I strongly suggest you post more details.
You also should make sure VS x64 is fully installed on your machine. I've seen a few problems there.
As pointed out by David, you should check for full installation of VS x64. The unresolved references may be showing up due to some extra inputs to the linker given in project properties, may be due to some specific 32 bit lib file that you may have included.