Did you consider looking it up on Mathscinet/Google Scholar/Zentralblatt and then order it through your library?

au:Mathieu, Olivier & ti:demazure & py:1986-1986 on zentralblatt gives

Mathieu, Olivier

Formules de Demazure-Weyl, et généralisation du théorème de Borel-Weil-Bott. (Demazure-Weyl formulas, and generalization of the Borel-Weil-Bott theorem). (French)

[J] C. R. Acad. Sci., Paris, Sér. I 303, 391-394 (1986). ISSN 0764-4442

but I also got this information with a google search.

Edited to add: Look at Willie Wong's comment to the accepted answer. The above reference corresponds to the question, but the actual long article is presumably

MR980506 (90d:17024) 17B67 (14M15 17B10 20G05)
Mathieu, Olivier Formules de caractères pour les algèbres de Kac-Moody générales. (French) [Character formulas for general Kac-Moody algebras] Astérisque No. 159-160 (1988), 267 pp.