To make an object disappear you need to set it size in all three axis to 0. Set a keyframe the frame before you want it to disappear where it is fullsize then advance to the next frame, set object size to zero, set a new keyframe (assuming you aren't using auto-key, otherwise it should set a keyframe for you). If you wish it to appear again note it's size before you zero it out and repeat the steps above but instead of making it size zero you'd re-enter it's original size(or whatever size you wanted). Is that clear?