Ahhhaa. Use non oily fingernail polish remover or rubbing alcohol to clean the heatsink base and the CPU. Wash hans thouroughly afterwards, the paste can be poisonous.
Use a rice grain-size of the AS-5 in the center of the CPU, put the heatsink down and with a bit of pressure, twist it osme to spread it. Then clamp it down, don't take it off to see if it spread out. it does.