when you open the wallet, side by side with the staking button there's a precomputing button, just click the grey area and it'll become blue.
enabled!
enabled!
Ah.. Thank you very much may be this will bring me some luck in the staking.
Guys, when I download new update, all synchronized datas will remain? Because after few days I am still on 30% due to my slow internet connection. 
Yes, all the synchronized data should remain intact. I would still suggest you to download the chain snapshots because they are much faster to download.