While in the HUD Designer click on the Export button and choose where you want the code to export to. Console prints directly to console (~) and Text saves it to a text document in garrysmod/data/hud_designer. Thats where you can find your newly-exported Lua code

http://wiki.garrysmod.com/page/GM/HUDPaint
All gamemodes will have a HUDPaint function and most of the time its under cl_hud.lua. The code in those functions is the same as the code this script exports, it works the same way. So you can find the gamemode in garrysmod/gamemodes and then once you find the HUD file you just have to put your code in there. Thats the simplist explanation of it. Or you could use a Hook instead of overriding default files

At the moment, this addon doesn't support stuff like health bars because it will take a bit of effort for me to code into this (I am not a wizard), but you can figure that stuff out with easy math.

Remember, this doesn't magically create HUDs. It only creates the code used to make HUDs