Can you share us a screenshot of your After Effects interface.
It's hard to tell like this.
Simply reply and add a picture using the icons above the text field.
I'm going to assume that you mean 'Markers' rather than keyframes. Because you say "*" and that's a shortcut for markers.
A marker is like a note you can place on a layer. See below:
If this is what you mean..
Try selecting a layer first (like "Planet") in the composition in the bottom and hit *
Or try ALT+* to create a marker.
Does this help?
If you mean adding keyframes by Animation menu -> Add Keyframes:
Try selecting a property of the layer. For example, pick the layer "Planet" again and press T.
This shows the property 'Opacity' in the layer panel. Click on 'Opacity' to select it.
Now you can add a keyframe by Animation -> Add Keyframe.
This is the first time I've opened a project in After Effects.
Thern by all means do read the help! You don't keyframe layers, you keyframe their properties. Without even understanding this basic concept, how do you hope to be able to pull of anything?