A quick google search turned up:
So it would appear that the express edition doesn't come with 64 bit
compilers, to get them you would need to install the Windows SDK.
I'm using Visual Studio 2010 Professional with the Windows SDK installed. So
that's probably why I don't have any issues with compiling 64 bit binaries.
I'm just a uni student so getting my application to compile as 64 bit was
just a "fun" exercise. It seems to work and my own tests pass, but I only
use a very limited subset of MySQL++ (I use ConnectionPool but not SSQLs) so
can't comment on everything.