I brought a game from istant,they gave me a code,how do i activate it?

At the top of the GOG website - hover over your account name, then click Redeem a Code.

But be aware that if you purchased a code from a third-party reseller, you're running the risk that it may not be legitimate (it could have been originally obtained with a stolen credit card, etc.) and GOG could remove the game from your account if the code is determined to be fraudulent.

Don’t know what istant is, but I would be very careful about buying keys. There is lots of stories of fraud and such like. If you want a game from here, but it direct from the store.

*points to the redeem code button*