From the trouble shooting guide of Apple's PowerBook Duo Service Manual

Troubleshooting/Symptom Charts/Miscellaneous Page 24

Can’t eject computer from dock

1 Unlock dock and press eject button again.

2 If no power to dock, insert dock’s key or small screwdriver

into square hole on side of dock and press to manually eject

computer.

http://www.retrocomputing.net/info/siti/apple_repair/www.whoopis.com/computer_repair/PowerBook_Duo_Dock.pdf