This is more geared towards new users that are running Windows. Hopefully it will save people some headaches.
Here's the easiest way that I've found to set up Code::Blocks and MinGW for 64-bit compiling on MS Windows...
I recently found this site: http://www.drangon.org/mingw/
Go there and grab this file (or newer): mingw-w64-bin-x86_64-20100709.7z
Open the mingw-w64 archive with 7zip, select Copy To, and pick a locating to extract the files.
Find the mingw64 folder you just extracted, right click on it and Cut/Paste it into a path without spaces (ie C:\). Full path should be like "C:\mingw64".
Open Code::Blocks and click on Settings->Compiler and debugger. Under Selected Compiler, click Copy. In the next box, type something like "MinGW 64-bit Compiler" and click OK.
Then on the Toolchain executables tab, I had to click the Browse button beside each option and browse to the newly created C:\mingw64\bin folder to set each option as follows (just changing the main path to the compiler didn't work):
Compiler's installation directory: C:\mingw64
C compiler: gcc.exe
C++ compiler: g++.exe
Linker for dynamic libs: g++.exe
Linker for static libs: ar.exe
Debugger: There isn't one for MinGW 64-bit yet, clear this and leave it blank
Resource compiler: windres.exe
Make program: make.exe
After a test compile, GnuWin32 File