To join a token network navigate to the "Tokens" screen where you can choose to either:
Click the "JOIN NETWORK" button next to the token which network you want to join.
Enter the amount of tokens you want to allocate and click "Join".
The amount you choose is what will be available for making payments and you can always add more funds later.
Joining a token network is an on-chain activity and will therefore consume some of your ETH.
You're now ready to make a first payment!
If you want to join the network for a token and the token is not displayed in the list of tokens it might mean that it has not been registered yet. In this case you can register it yourself.
Click the upper right "+" button.
Enter the address of the token you want to register and click "Register".
To join the network of your newly registered token follow the steps for joining an existing token network.