In order for you to see the Sinhala words, you must have installed a Sinhala Unicode font on your computer. On Windows Vista, there must be one already. I find these instructions at Sinhala Bloggers' Union easy to follow and to the point.
Sumihiri is a set of rules that can be used to write Sinhala using an English keyboard. The rules have been designed to make typing and reading easy. The rules are explained in the two side panels in the 'Sumihiri to Unicode Converter' page. That tool allows a direct translation of sumihiri text into the emerging standard of Sinhala Unicode. To learn about the thoughts behind the desing of sumihiri, read this.
In its current state, the tool stops translating your text when it encounters something which conflicts with the sumihiri conventions. By inspecting the spot where it stopped, you can easily correct and convert again.
This is currently possible with Linux. On Windows, we need someone to write a keyboard driver for sumihiri. But you can always use the 'Sumihiri to Unicode' converter to write Sinhala, then copy and paste into whatever application. (That's how I write Sinhala on Windows machines.)