In: http://clanlib.org/wiki/VisualStudio2010GlobalIncludes

If you are using 64 bit versions of ClanLib, you need to change the Platform combobox to x64 and modify the Include and Libs directories there as well.
Either you forgot to do this, or you used "C:\Source\ClanLib\lib\win32" on the x64 target, instead of "C:\Source\ClanLib\lib\x64"

Or maybe you used "C:\Source\ClanLib\lib\x64" on the win32 target

Personally, I would not use x64, unless you require the x64 features. You application is more portable.