What you have to do is assertain what type of face you're dealing with and convert it to the coresponding geometry type, for example, in C++, if it's a type of 'kPlane' (which you can get by using the getNativeGeometry() function), you can cast it to a AcGePlane object and query it from there.
There is examples of getting of brep data in the ARX SDK, might be a bit of a stretch though if you are new to C++ and ARX.
How to do it in C#/VB.Net is another story though
there's a post on Kean's blog about it which might help.
A quick hack though is to loop through the face's edges, find out what the winding is (this is important as edges can produce wrong normals depending on their direction in relation to the last edge) and find 2 edges that comply with the winding and get their cross product which is the face's normal.
Easy eh
Sorry I can't be more help at the moment but there's plenty of clues and tips in there to ponder, cheers.