You wanna post the bit from the help file?
Happy to, Alistair! There was no printed manual (to take advantage of the whole on-line concept), so here's the table of contents from the THEXDER.HLP file:
The Thexder Saga
Getting Started
Logging-on
Difficulty Levels
Setting the Keyboard Controls
Optimizing Game Performance
Weapons and Equipment
Saving Your Game
The Menus
File
Window
Options
Help
Network Play
Problems and Solutions
Credits
Thexder Extras
Secret Stuff
Play Hints
Maps and Solutions
Level Editor
More Levels
and here's the Thexder Extras section:
Available January, 1996
A supplemental on-line Help file that contains all the information you need to get through each level of Thexder plus an editor to create your own new levels to challenge yourself and your friends plus lots of new map levels to play. A real value and a great addition that makes Thexder even more fun! Included are the following:
Secret Stuff
This part of Thexder Extras gives you the secret codes to increase your ammo, equipment, energy charges, and energy cells (extra lives). It's as easy as typing phrases from your keyboard. Now you can get past those levels that are just a bit too tough.
Play Hints
Having trouble getting past one of the techies? Can't seem to avoid that lava pit? Can't find the hidden path to a new location? Use this section of Thexder Extras to find the most efficient weapon to eliminate that techie, the best form for Thexder in some locations, and where to find hidden weapons and equipment. Find out each techie's pattern of movement and when he is vulnerable to your attack.
Maps and Solutions
The third part of Thexder Extras illustrates graphically a successful path through every level and shows hidden shootaways and foreground cells. Find the locations of hidden ammo and equipment, and read hints on how to get to them.
Level Editor
Create new levels for any world to surprise and challenge your friends. This is the development tool used to create and decorate all of the levels in Thexder. Have access to the special tiles for walls that you walk behind instead of bumping into, shoot-away tiles, platforms, switches, lava, acid, caves, conveyers, and invisible floors and walls.
More Levels
Brand new levels have been created for those who just can't get enough of this robot. New ideas, new challenges, and new puzzles are all available to challenge you at any difficulty. Explore the new ice caves, race along the girders of Geigerus, or find your way through the the caves of Chaox. An absolute necessity for all Thexderites.
How to Order
Cost of Thexder Extras is $14.95 plus shipping and handling.
For more information call 1-800-757-7707 or contact Sierra via World Wide WEB at
www.sierra.com<< " World Wide WEB " -- HA! Were we ever so naive?

>>
There are NO working cheats/codes to be found on Google. The only one that everyone refers to [Start game with Caps Lock on / during game, press Esc, Y, Esc for full energy] does not work -- at least, not on my machine. If there were all sorts of cheats and codes and hints and tips, as claimed by this file, I figure someone somewhere would have uploaded the data to at least one game hint website. Or someone would have uploaded the level editor somewhere...
...So, I'm guessing it was never released, or no one bought it. One forum post did mention that the poster tried too late to purchase it from Sierra -- Sierra no longer had it available by that time.
Gary