First of all, make sure the game is really loading your mod. It should give you an error message, but I've never trusted these electronic devices, so whenever I test something, I always change the <fScale> value of the Settler unit to something exorbitantly high, like 40.0 or above. Once the mod is loaded, you should see a gigantic Settler unit, or something went wrong.
If something went wrong and you got no error message, make sure the folder you put your practice mod in, is set up exactly like the folders of the game - that is, you should have it like this example:
-- Sid Meiers Civilization
------|-- Mods
-----------|---- Practice Mod
--------------------|-- Assets
--------------------------|-- Art (if you have any modified art)
--------------------------|-- Python (if you have any)
--------------------------|-- XML
------------------------------ |-- Art
------------------------------ |-- GameInfo
------------------------------ |-- Units
-------------------------------------------|-- CIV4UnitInfos.xml
If you set it up incorrectly, and just put the XML/Units folder directly in the 'Practice Mod' folder, forgetting all about the 'Assets' folder, the game won't load your changes, cause it's not looking for an XML folder in the Practice Mod folder, but in the Practice Mod/Assets folder.
Hope that's somewhat clear - a picture probably would have told a thousand words, but I'm a bit lazy now
