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/ (transaction fees of each trade is sent to the fee pool to be claimed by staked SNX holders).
Rewards can be claimed weekly from the 'Claim' button in Mintr.
Before claiming rewards, check your Collateralization Ratio status. If your Collateraliszation Ratio status is below the target collateralization ratio, you will not be able to claim rewards until you have adjusted it.