You have to buy them in the reality engine (unlocked after getting singularity) then you click on the new icon near the 2x speed boost one (should be near the bottom left corner of the screen, doesn't appear in the reality engine)

Have you bought the nano buyer upgrade in the reality tree (unlocked after 3-4 nanobots)? If you have, then activate nanobots then click the icon again and it goes into buy mode.