Basically I need to know if Foolish Burial chooses a target at activation. If I chain Macros to Foolish, does my opponent choose his target before or after I activate Macros?

They cannot target anything in the deck, You choose at resolution of the card effect. So if they intend to send Treeborn Frog to the grave and you chain Macro Cosmos to the activation of Foolish, then they could send whatever they want out of play.