What if you used beans (or small bears or buttons or…) and had her set out the one she knows – your example was 4 + 5, I believe. Then have her move three from the 5 to the 4 to get 7 plus 2 and move 4 from the 7 side to the 2 to get 3 + 6 and so on. The request to move from one side to another should be verbal for the first, few, then ask her to say some and come up with the equation. This just seems to help those kinds of ideas click for some kids.
BTW, I don’t know what it is about moving from the blocks in MUS to a non-math related manipulative, but that may help. I think it has to do with the tactile – liking the smooth bean shape over the pointy square-ness. However, move from beans (or whatever) alone, to arranging the beans and picking the correct MUS cube/rod for each pile (ie- 4 beans + 5 beans should be represented as well in the correct rods below), then move to rods alone. This, I think, helped with the quicker color recognition in later levels when she was no longer interested in the beans.
Incidentally, I agree with whomever mentioned setting the math book aside for a bit. Play with your math for a bit before going back to writing it. See if it helps. 🙂 If she’s a late bloomer in other areas, there’s no need to fuss about this. If she just learned to tie her shoes, but when she did, she *GOT* it, she was likely very proud of her accomplishment. If you push the math and she’s not ready to grapple with the symbol on the paper being equivalent to the sets of beans or cubes and then translating that back and forth to get the answer, she’s going to feel frustrated with math which may set up a road block later on – which I’m sure you know. Better to back off a bit and let her breathe the physical-ness of math then force the sybols equal an amount before she’s ready to deal with it. You’re a good mom who wants the best for her. It’s ok to say that the best for her, right now, is to wait. 