To add a build command.
Things to do.
First
Make your Custom Tower (the on that is to be built)
Make your Custom Builder (the one who will create the Tower.)
Now, to add the Tower to the Builders command card you have to do the following.
Find the ABILITY (SCV - Build, or Probe -Build, or Zerg Morph) depending on the race building graphic you want it to build with. Duplicate OR alter it. depending on your need. (Do not check any other of the items in the Duplicate window as you wont need duplicates of the units themselves, the movers or the build effect. The defaults will work just fine.)
In your Duplicated or Altered Ability, find the Field 'Info+' and open the field editor. You will see a list of the buildings this ability is allowing to be built.
Remove all of them for your custom builder. (you actually have to right click and MODIFY each entry, and delete the items under 'requirements' and 'unit' to have a unit be removed completely.
Then simply add a new unit. This is also where you set how much it will cost and how long it will take to build and even its requirements.
Add your Custom Tower to this list.
Ok. now that the ability is done, we need a button for that specific tower.
Open the BUTTON data editor.
Duplicate or Alter the 'Build (Structure Here)' button, or whatever other button you so wish to use. Change the name of the button and the tooltip here. Also set the Hotkey, Make sure no other buttons in the same command card have the same hotkey, otherwise you wont get the desired effect.
Now go to the UNIT Data editor.
In the units Abilities+ field Add the Ability you just made.
In the Command Card+ field click where you would like the button.
----READ IF YOU WANT TO HAVE YOUR BUILDINGS IN A SUBMENU----
Click the little green X button on the right. And add the button for BUILD STRUCTURE (or any custom one, if you so desire). Then set that Buttons command type dropdown to SubMenu and choose the card number you desire your custom towers to be listed in.
Then open that command card (1-4) at the top of the command card editor.
-----SKIP HERE OTHERWISE----------------
Select the location for the first Tower Build Button.
Click the little green X on the right. This will allow you to choose your newly created button.
Now Under the command type dropdown in the command card editor. Select ABILITY.
If you dont want any special requirements to use or show the button, leave REQUIRMENTS blank.
Under the ABILITY Dropdown in the command card editor, select the BUILD ability you added to the unit.
Then, it should populate the available units/structures in that build ability. Select the Tower that matches the button you put.
There you go. It should show up in the game and also cause the unit to be built upon use.
Hope it wasnt too confusing. I think I got it all, if I missed anything Im sorry.