The resource/time costs of units are to be found in the Abilities section, under their respective production building's Train ability. For example, Terran Infantry costs can be found in the Barracks - Train ability (in the Info fields).
Exactly, just double click on the "Info +" Data under Abilities->Barracks - Train and then double click on the ability you want to change the cost of (scroll down...)
Any idea how to change the requirements of Structures? in the same window that you edit resource costs it shows what it requires, but there doesn't seem to be a way to edit it from there.
Let's say you want to change the requirements for the "Missile Turret".
In the Data Editor go to Abilities -> SCV (Build)
Double click on the "Info +" field
Double click on "Build Missile Turret"
Now a little down the way you see "Requirements".
You can click on the little red x below the list to clear all requirements.
OR you can choose another requirement available.
OR you can write your own requirement and choose it here afterwards.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.