To get it to read an XML file with a new tag you have to modify the schema file for that XML file to include the new tag. So you got the easy part done.
That just gets it to load without errors. To get it to actually do something with the new tag you have to mod the DLL so that it actually does something with the data (like save it into a new variable in the object's info class and then actually use it wherever appropriate in the actual game code). The game does not magically know what you want to do with some value that you have added to the XML, you have to make it use it the same way that it is programmed to know what the existing tags do.
If you are modding something that does not have its own custom DLL this is easy, you just use the one from the mod you are merging in (although this may bring along more stuff that you really wanted). But if the mod you are starting with already has a custom DLL, then you have to merge the functionality into that DLL by merging in the appropriate stuff for the functionality you are trying to get from the 2nd mod's .h and .cpp files into the 1st mod's .h and .cpp files and then compile a new DLL. To do this you have to set up the right versions of the software (Civ was built using a version of C++ from 2003 or so and it has to be the same version) and such. Some people have difficulties getting this to work, for others it goes smoothly. Likewise, once everything is set up correctly so that you can compile an unmoded Civ DLL, some people copy features from one mod to another without difficulties but for others it does not go so well. See the appropriate Tutorial thread(s).
