Let's say one of your attributes is strength, with a baseline of 10. You could put this in your "Startup" passage,
(set: $strength to 10)
If you want the player's strength to change when clicking on links, you could do this,
(link: "You spend a couple hours doing heavy farm work.")[
(set: $strength to it + 1)
(go-to: "Go back to town.")
With a resulting total of 11 strength. Then use an if-statement to check how much strength the player has for ability checks. I'm sure there are better ways of doing this, but this is what I make do with. Note that you might not get those neat little grey arrows that connects passages in the overhead story view. I certainly don't and I would love to know how to fix that if someone more experienced could chime in.