C++ is strongly typed language, so you the types have to match. In the first try your supplying object of type GLuint to a fucntion that takes void*. In second example your supplying pointer to GLuint to the same function.
The only way to make this work is to supply GetEnvironmentMap function with something of Type void* (or something that may be implicitly cast to it).
EDIT: If you're using OpenGL, GetEnvironmentMap function will gladly accept pointer to GLuint as first parameter, but it has to be casted (to void*) and it has to be valid. The code that might help you is
GLuint myEnvironmentMapTex;
silverLining->atmosphere()->GetEnvironmentMap((void*)&myEnvironmentMapTex);
Void* is a pointer that points to some memory that no-one knows what holds (it looses any type information - 'void==no type'). Read more here: pointer to void