If you are using the main C++ distribution of wxWidgets, Feel free to ask any question related to wxWidgets development here. This means questions regarding to C++ and wxWidgets, not compile problems.
That's not proper code. _("test") is not a wxString, it's a char set, which can be converted into a wxString, but the resulting wxString is const
What I recommend doing if you're new to C++ is to first try to compile your program all together (no dlls,) that way the compiler will tell you if you have parameters that don't match up or something, then pull out what you want into a dll. That's what I do and I've been using dynamic libraries for years.
If it was std::string, I'd recommend not to use it, because of the infamous DLL allocation pitfall (you must not allocate objects in EXE and free them in DLL or vice versa). I'm not sure whether this applies to wxString too, but probably it does. Try changing wxString to const wxChar* in all of your argument lists in the class member functions. You can still use wxString internally in the DLL, but probably not as an argument for a member function that can be called from the EXE.