If you mean the client, then follow the instructions bellow: If you're in the same network:
1. Right click on your friends name
2. Click "Add as friend"
3. Click "Yes" in the confirmation box
4. Done !
If you're not in the same network:
1. Click the "Friend List" button from the top right corner of Tunngle client
2. Click "Add Contact" from the top of the friend list window
3. Search for your friends nickname and double click to add as friend .
4. You'll get a confirmation box, click "Yes"
5. Done !