Only problem i'm seeing with the tuner is, when you drop a resource and your city uses it, the city wont allow you to build specific buildings towards that resource, like for silver/gold, you'll get the happiness but your city cant build the Mint building for some reason...
Try using the tuner to add the mint to your city OR try building it outside the city radius then buy the tile in game.
This is a bug to be reported up to Firaxis games. Their modding forum is at this link:
forums.2kgames.com/forums/forumdisplay.php?f=89
Using your directions I can add Techs. I think I've figured out how to add improvements and resources but not units.
For units you just have to select the correct civilization in the window listing these, then select the unit you want in the unit list. After that it's just a matter of enabling the unit plopper and clicking away.