c++: definition of dllimport function not allowed, building with visual studio 2010

"User macros" in VS property sheets have nothing to do with preprocessor macros. Put TRIALDLL_EXPORT into the property sheet's section C/C++ > Preprocessor > Preprocessor Definitions

"User macros," which can only be defined in property sheets, allow you to create your own "variables" usable in Visual Studio properties, similar to the built-in ones $(TargetName), $(SolutionDir) etc.