Hello! You need to open the dev menu by:

• Go to Settings
• Scroll down to Tonkeeper's icon.
• Tap 6-7 times (fast enough!)
• Dev menu opens
• Switch to Dev mode
• Return to the Main screen.
• Press on The Wallet button.
• Choose "add a new wallet"
• A wild option to add a Testnet Account appears.
• Add a testnet account
• You are good to go!

Have a good day!