Ok, now please let's not get a mess in this thread. The original poster posted a bug report and your problem seems to be how to set an effect.

For applying an effect to a material please read the manual under the link that I gave above. Your function above won't work as a replacement for effect_load(). An effect is a .fx file on your hard disk that must be really loaded. What you're doing was just setting the file name as a text string. Please use effect_load() and if you then have a problem with the decal effect, please post again. We might then need your project for reproducing the problem.