Go to the Bot Store, find the bot you want to add to your account and click on "Add to Client" - The green box that appears when you hover over each bot.
Close Runemate, re-open it and the bots should appear in the list on the left hand side of Runemate. Alternatively, you can click on the refresh circle at the top of your bot list in Runemate. You can have as many bots added to your Runemate account as you want.
As a free user, you can only run two instances (sessions) at a time. Eg: 1 account botting woodcutting, 1 account botting fishing would be your maximum amount of sessions. As a supporter, you can run 1 extra instance for every $1.00 you donate.