Well, you go to the Data Editor. Then to the Units section. You're wanting to edit the Barracks, so go find that and click on it.
Then, to allow it to train a new unit, you'll need to double-click on the Ability - Command Card + section, and add a button for training Siege Tanks. Also, you'll have to actually allow it to train the Siege Tank; to do that, go to Abilities, find Barracks - Train, and edit Info +, right-clicking on one of the unused Train slots to add your new unit.
Well, you go to the Data Editor. Then to the Units section. You're wanting to edit the Barracks, so go find that and click on it.
Then, to allow it to train a new unit, you'll need to double-click on the Ability - Command Card + section, and add a button for training Siege Tanks. Also, you'll have to actually allow it to train the Siege Tank; to do that, go to Abilities, find Barracks - Train, and edit Info +, right-clicking on one of the unused Train slots to add your new unit.
If the icon doesn't show up, make sure you're editing the Ability Command Card as well as the plain Ability section.