There are several ways to copy and paste keyframes:
1. Select the "source object" and go to the animated parameter in the Inspector (position for example).
2. Click and drag the parameter name you want to copy to the destination layer in the layer list via drag and drop. The values (including keyframes) will be copied to the appropriate channels.
OR
1. Select the source object.
2. For each channel that is animated (position, rotation, scale) go to the far right side of the parameter in the Inspector and select "Show in keyframe editor."
3. Select all the keyframes you want (I use a simple click and drag to create a box an select all of the keyframes) and then copy (command-c).
4. Now select the destination layer, position the playhead where you want the keyframes to start from and paste (command-v).
http://help.apple.com/motion/mac/5.0/en/motion/usermanual/#chapter=12%26section= 12
If you just need to match the animation of another layer (Position, Rotation, Scale) try using the Match Move behavior.
http://help.apple.com/motion/mac/5.0/en/motion/usermanual/#chapter=22%26section= 7 (See the section: "To apply the transformation of a source object to a destination object")