ArcObjects can only run in a 32 bit process because they are built as a set of 32 bit DLLs.
32 bit processes can run on 64 bit machines
32 bit processes cannot call functions in 64 bit DLLs
64 bit processes cannot call functions in 32 bit DLLs
Compile your .NET executable for 32 bit processes only.
This can be avoided by selecting x86 CPU rather than 'ANY CPU' when compiling. (advanced compiling options)