This is a little wierd. I used the installer from the www.tremulous.net to install tremulous, and there is no UI folder inside base to add a custom HUD.
You have to make the folder yourself. In windows right-click -> new -> folder and name it ui.
oh, okay.
'nother question. The HUD readme mentions an autogen.cfg in the base folder, but it doesn't seem to exist.
autogen.cfg is created the first time you open Tremulous.
To see where it went, open Tremulous, bring down the console (use the ~ key) and type:
/fs_homepath