I'm not sure what the problem is, but try opening up what ever software instrument is running your MIDI sound. I think (in the case of the GM Piano) it's the ESX24 sampler. You should be able to change the instrument through the sampler, or by clicking the Media button on the right side of the arrange window.
As for the effects, try holding down on the effect button in the track in mixer view. Your drop down menu of effects should pop up and you should be able to select a new effect....
if that doesnt work, i dont know what to tell you. Just keep working at it. It's a pretty simple program