Select your head mdoel and return to the modify tab. Go to vertex mode and begin adjusting the shape of the eye so that it deforms around the eyeball better. Go back and adjust the position and size of the eyeball if you have to. Continue this until you are satisfied with the shape of the eye.
When you're satisfied with the position of the eye, duplicate the sphere for the eyeball Select it, have the move tool selected, hold down shift and click on the move arrows, but don't drag anywhere. A dialog will appear asking you to name the new object. You can if you want, it doesn't really matter at this point. Now, with the move tool still selected, go to the bottom of the screen under the timeline where the X, Y, and Z axis numbers are. Put a "-" sign in front of X if it's a positive number. If it's a negative number, remove the "-". If this doesn't move the eyeball to the other side of the face, undo it, and try changing the Z value instead.
Now the eyeball should be in the correct location, however it isn't rotated properly. Choose the rotate tool now. You will probably need to negate two of these values to get the sphere in the correct position. Try them each until you get it right.
It's a good idea to check back with the front and side references and views every now and then. Right now since we've got most all of the big things worked out, it's a good idea to just go over the entire face and fix anything that you don't like. Make adjustments to chaks, debth of eye sockets, chin, etc. Just fix anything that doesn't seem to look quite right in perspective view. Pay attention to the flow of the edge loops. Anything that doesn't seem to be flowing in a smooth curved line, could probably be changed or adjusted to work better