I am having the same issue. Sometimes it works, othertimes I die. I don't think it's the trainer portion per say, but maybe you have to activate it at a certain time? Or when you change screens? I don't know but I too am thinking I'm missing something here.
Any tips would be apprecaited. I hit f1 at menu, hear trainer activated. Then in game I hit num 1 and hear activated. Oddly, sometimes I hear deactivated and then I keep hitting it until I hear activated. Does infinate health need to be activated only once or does it reset?