I'm seeing nothing that strikes me as obviously wrong with your code. Are you sure that passing transparentPixal to the function doesn't work and passing al_map_rgb(0, 255, 114) to the function directly DOES work? That is the only thing that is puzzling me. I'm not currently at my home computer, so I can't test the code myself. If you look at transparentPixal in your debugger after asigning the value to it, what does it say? And in case you have access to the Allegro source while debugging, if you step into the call
al_convert_mask_to_alpha(bmp, al_map_rgb(0, 255, 114));
What does the color argument say when looking at it in the debugger?
Also, what OS are you on, what version of Allegro are you using and which compiler did you compile this with?