You may find yourself in a twisty maze of #ifdefs. Or you may be wondering why your macros aren't working.

I have these lines in my header file: 

#define MM_BUSY     0x0001
#define MM_IDLE     0x0002

but when I try to use them, I get errors. 

sample.cpp(23): error C2065: 'MM_BUSY': undeclared identifier
sample.cpp(40): error C2065: 'MM_IDLE': undeclared identifier

Any idea why this is happening?

First, make sure the compiler even sees you. Notice that for macros, generating a preprocessed file doesn't accomplish anything since #defines don't show up in the preprocessor output. (They are preprocessor input.) What I do is use the #error directive. Add it to the header file and recompile. 

#define MM_BUSY     0x0001
#define MM_IDLE     0x0002
#error Did we get here?

If you get 

sample.h(80) : error C1189: #error :  Did we get here?

then you know that the line is indeed being compiled and that somebody after you is doing an #undef MM_BUSY. If not, then you get to investigate why the lines in the header file are being ignored. For example, they might be hidden by an #ifdef, or (if you're using Visual Studio with precompiled headers), your #include directive might be ignored due to an overriding precompiled header directive. You can scatter #error directives into other parts of the header file (or other header files) to narrow down why your lines are being skipped.