mat_shadow.alpha controls alpha of dynamic shadows, to change brightness of static shadows you should change some parameter in map compiler(shadowmap brightness or something like that)
Modify ambient in the map properties in wed. For more informations look into the manual under ambient. If you use A7 than it should be lightmap gamma (mapcompiler).
Thanks for all of you , the lightmap gamma option was the solution , but I wanted that the unlighted regions to be less darker , but the whole regions became brighter , I guess thats the best result for gamestudio