Ok, step by step instructions on how to make an upgrade with 255 levels, with each level increasing a (default) Marine's damage by 1:
- Go to the Upgrades tab in the Data Editor.
- Right-click anywhere in the upgrades list and select Add Object.
- Give it w/e name and ID you want, base it on "Default Upgrade", and the rest of the boxes don't really matter. At the bottom, select "Set to parent value" and click ok.
- Turn on table view (just so the rest of these instructions make sense).
- In your new upgrade, double-click on the field labeled "Effects+". A pop-up window will appear.
- In the box at the top, right-click and select "Add Effect".
- For the new effect, make it of type "Effect".
- In the "Object" field, select Marine - Guass [sic] Rifle (Damage)".
- Now in the box labeled "Field" select the very first line that's labeled "Amount".
- For "Operation" select Add.
- For "Value" put in 1.
- Now exit out of the "Effects+" pop-up and go back to your upgrade, now double-click on the field "Affected Unit Array". A pop-up window will appear.
- Right-click in the box and select "Add Value". Now make the new value equal to "Marine".
- Exit out of the "Affected Unit Array" pop-up.
- In the field labeled "Max Level" input 255.
Now add that upgrade to w/e research ability you want, give it a default button, put that ability and button on a unit, and there you go.
If you want the upgrade to also increase the level of the unit's attack (that number that appears in the corner of the weapon icon in the UI when you select a unit), then in the upgrade add another effect of type "weapon", and make it add 1 to the field "Level".
I've attached a picture of what the upgrade should look like in it's finished stage (I didnt' use table view cuz it's hard to show everything in one screenshot in that mode).