Summary

This is an eyeball with left/right, and up/down manual controls.
It uses a ball and socket joint that snaps together to allow eye movement within a socket. Strings are attached to the inside of the eye from within the control box and through the neck which allow you to make the eyeball look around by turning the knobs.

Video: http://youtu.be/Emg2zwOq7Ck

Instructions

All parts are printed without supports if you orient them properly.
Neck parts all have a flat end with button holes. Make sure that end is flat on the build surface.
Print the socket and eyeball with the open end facing up.

I used a scaling factor of 500% and printed with PLA on a Replicator 2 at 0.15mm resolution.
Besides the printed parts you will need some good, thin string and some super glue.

Here are the assembly instructions (see photos for reference).

Notes: the string can be tricky to handle so you might want to get a third hand to help. You may also have to shave or sand the stems of the eyelets slightly so they fit into the rectangular holes in the eyeball fairly easily.
The ball and socket snap fits between the eyeball and socket.
This can only be done once. You will never get them apart once they are together without breaking it!

1) Cut 4 pieces of string about 1.5 to 2 ft long
2) Thread the 4 pieces of string through each of the 4 "button" holes on the long neck. The flat part with the button holes will be the bottom.
3) Continue threading through the middle neck piece and to the end neck piece that connects to the eye socket.
It is important to not let the string cross or get twisted. Try to orient the button holes so there is a top, bottom, left and right hole. Keep them aligned and pay attention to how the neck bends.
4) Continue threading through the eye socket.
5) Carefully glue these pieces together in the right orientation without the strings crossing each other, being careful not to get any glue on the string. Once glued I recommend that you tape the bottom end of the strings so they can't accidentally pull through the holes. If any do pull through then you'll have to tear it apart and start again.
6) Thread each string through a small eyelet and tie a big enough knot in the end to stop it from popping back through the hole (use a dab of glue on the knot to keep it snug on the eyelet).
7) Push the eyelet only partially through each hole on the inside of the eyeball. Pay really close attention to the order of these eyelets so again the strings are in the right order. Triple check them as it is so easy to get it wrong! Remembering that the inside of the eyeball will connect to socket with the top, bottom, left and right eyelets oriented correctly.
8) Once you are triple sure of the orientation and connections put a little dab of glue on each eyelet and push it all the way through the hole. Do this for all four.
9) Use nail clippers and snip off the stems of the eyelets that stick through the eyeball.
10) Here's the critical, nerve-racking part where you snap the eyeball into the socket. You need to make sure the eyeball is in the right orientation and the strings are pulled so that they are fairly straight and NOT crossing, or caught in the ball socket. Once you know the ball is mated to the socket, snap them together nice and firmly. You might want to shake it to make sure nothing broke.
11) Glue the neck onto the box top with the strings through the bottom.
12) Arrange the knobs and levers so that they are oriented 45 degrees as shown in the picture.
13) Tape the eyeball still so it is centred and not moving around.
14) Pull the strings so there is no play in them and guide them through the big eyelets to the appropriate levers (top and bottom go to one lever, while left and right go to the other).
15) Thread through the holes in the levers and tie the strings.
16) Trim the strings, glue the knobs on, glue (or screw) the box bottom on and you are done!

Animatronic Eyeball with Manual Controls