To use a Knowledge point instant you must have an Ancient Wonder or use them to donate to someone else's Ancient Wonder.. You just have to enter the wonder and click the little icon of the book and it shows you the available instants you can use
To use a Time reduction instant just click the building. province scout, manufactory or troop que and the time remaining will be highlighted in a green box.. once you click it, a list of time reductions instants you own will show up to choose from