To earn rewards you need to stake your SNX as collateral and mint sUSD using Mintr.
Rewards are generated from trades on https://Synthetix.Exchange/ (0.30% of each trade is sent to the fee pool to be claimed by staked SNX holders).
Rewards can be claimed weekly (for up to 2 weeks) from the 'Fees' screen in Mintr.
Before claiming rewards, check your Collateralisation Ratio status. If your Collateralisation Ratio status is below 750%, you will not be able to claim rewards until you've updated it to at least 750%.