Make sure you've actually made frames--if you can make tweens it means you haven't filled out the frames in between.

Select which layer you want to add frames, hit F6 on your keyboard until you have the amount of frames you want.

You can also go to insert>timeline>keyframe.

Once you've made an image, go to the next frame and make another image. Another thing that might help is the onion skinning button, located on the bottom of the timeline, right above the stage. Hit that button to see what your previous frames are doing.