Select the gear icon at the top of the RushWallet window to see the Settings menu:
Select Export Keys.
This will show you your Brainwallet in encrypted form, along with your private key. If you need to decrypt your Brainwallet, you can do so at bitaddress.org.
Disclaimer: Please make sure you do not share your private keys with anyone, including Jaxx support team, as if someone gains access to them, they will also be able to manage your funds.