The Tai Heritage Pro fonts are encoded according to Unicode, so your application must support Unicode text in order to access letters other than the standard ANSI characters. Most applications now provide basic Unicode support. You will, however, need some way of entering Unicode text into your document.
The Keyman website has keyboards for typing the Tai Dam and Tai Dón languages in the Tai Viet, Lao, and Latin scripts. (There are no keyboarding helps or utilities packaged with the font.)
Tai Heritage Pro is designed to work with two advanced font technologies, Graphite and OpenType. To take advantage of the advanced typographic capabilities of this font, you must be using applications that provide an adequate level of support for Graphite or OpenType. See System Requirements.