link libspatialite-2.dll 
and add in the bin folder: 
libfreexl-1.dll, 
libgcc_s_dw2-1.dll, 
libgeos-3-3-1.dll, 
libgeos_c-1.dll, 
libiconv-2.dll, 
libproj-0.dll

to load the spatialite library in the code:

            command.CommandText = "SELECT load_extension('libspatialite-2.dll');";
            command.ExecuteNonQuery();