Here is the list of environment-mixing restrictions within which you must operate when developing plug-ins with Version 9 and later of the 3ds Max SDK:

• Microsoft provides an OS-level compatibility layer, called WOW64, which allows 32-bit applications to run under 64-bit Windows; but it cannot be used in any other context, such as allowing 32-bit and 64-bit components of a single application to cooperate with one another.
• You can run 32-bit systems on 64-bit machines, but you can't do the opposite (64-bit systems on 32-bit machines).
• You can compile 32-bit and 64-bit plug-ins on both 32-bit and 64-bit computers using Microsoft Visual Studio; the compiler is a 32-bit application that runs in both environments.
• You can only run 64-bit 3ds Max on a 64-bit machine running 64-bit Windows.
• You can only use 32-bit plug-ins with the 32-bit version of 3ds Max.
• You can only use 64-bit plug-ins with the 64-bit version of 3ds Max.

You can simplify your 3ds Max SDK work environment by running 64-bit Windows on a 64-bit computer, building 32-bit and 64-bit plug-ins on the same machine for your users according to their needs.