Click the button for the network you wish to add to Keplr. If the network hasn't already been added, Keplr should popup and ask to add it.
If it doesn't, refresh the page or check if you already have it at the bottom of Keplr's network list.

If you have issues, restart your browser or remove and re-add the network.
To remove a network, select a different network, then click the X on the network you want to remove.

Secret Network