Well the button can be anything. Under the unit you need to make sure the units has the ability "research-whatever" and that the upgrade you want is listed unt the "research-whatever" skill. Then you need to make a button which is for the "research-whatever" ability, then specify in the next pull down what part of the "research-whatever" ability you want to be researched. For example there is only 1 scv-build skill and it can hold 30 buildings. Its not enough to tell it SCV build on the command card, you need to make sure the sub selection is picked too.
Lol, let me try to clean that wall of text up for you...
So basically EVERYTHING in star2 is an ability.
When you make a button, you need attach an ability to it.
So:
-You take an ability that is the "research" ability.
-You attach the upgrade to that ability.
-Make a button on the unit you want.
-Give the unit the "research" ability.
-Attach the ability to the button.
-Attach the upgrade to the action of the button.
Button -> UpgradeAbility -> Upgrade
So that when you press the button, it activates the ability, which activates the upgrade.
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That exactly what Dangime said, but how he said it confused me, so I rewrote it, hope that is helpful.