There are 2 ways of adding the Step Network to your wallet:
Adding Step network automatically
Connect your wallet by pressing 'Connect wallet' button on the top right
Once connected, you will be asked to allow adding the Step Network
Switch to Step Network
Adding Step Network manually
Check Step Mainnet parameters here
Open your MetaMask wallet and click on the networks dropdown
Press 'Add network'
You will be redirected to the new page. Click on 'Add network manually' and fill in the network parameters from the ones you copied in the first step
Make sure that you switched to Step Network in order to use it