How to pad a structure size to 32-bit?

My game book using C++/win32/dx suggests to either pad my structures to 32-bit manually, or by using a macro (easy way). Since each structure would be padded to 32-bit, this would definitely increase speed. I looked around the web and the docs, but I don't see this macro he briefly talked about. Does anyone know what it is

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How to pad a structure size to 32-bit?