The codes in the manual were only for the CD version and its copy protection that appeared at this point. If I remember right, GOG's version doesn't have it. Just jump into the pod, write down the green symbols on the keypad display (you need them later) and enter whatever you want two times and you're good to go ;-)

If you actually do get the copy protection, you see two symbols that you find in the manual's table. Enter the according letters from the X and Y-axis for both symbols (2 + 2 letters) and the pod is unlocked.

If you fail to write down the time pod codes, you'll find that many internet posts will tell you you're screwed and have to start over. Not true, there's a file you can download (debug patch) from spacequest.net which will allow you to simply click on Roger's left shoulder to select your destination.