hmm. i think instead of trying to key multiple balls at once, we'll work with just one to start so you can get accustomed to keying.

1. start a new scene.

2. make a sphere.

3. in the timeline, go to frame 0.

4. select the sphere. in the visibility attribute, enter a value of 0 (zero) and then right click on the word "visibility" and choose "add keyframe". the sphere should still be selected, but now invisible.

5. in the timeline, go to the frame where you want the sphere to reappear... for this exercise, choose frame 15. change the visibility attribute to 1 (one). your sphere should appear again. again, right click on the word "visibility" and choose "add keyframe".

if you scrub the timeline now, your sphere should slowly fade in from frame 0 to 15.

6. in order to make the sphere appear in ONLY frame 15, go to frame 14.

7. repeat step 4.

after following these steps, there shouldn't be any problems, and your sphere should only be visible from frame 15 and on.

if ya got any more problems after this, lemme know.