[zajako]

Thats cause you can't put it in post bits.

Postbits use a function and functions only carry globals recalled.

To fix this in admin/functions.php
find:
function buildpostbit(

shortly under that you see a line that says
//users
global blah blah blah

just add $eggshow before the ; on that line

But this will show an egg in every post not just one.

If you want it to choose a random post ignore all that ijust said above then instead of having the coding in the global put it in the postbit function under the globals

If you do that be sure to set the random lower.