If you are using aion-v0.1.8 or if you want a permanent id (used by peers), to connect to the Aion test network you need to first modify your configuration file to have a new personalized id.
Download the ID generation script generateId.sh here. Add executable permissions to the script.
chmod +x generateId.sh
Run the script.
./generateId.sh
Copy the output.
Navigate to the config.xml file in [aion_folder]/config/config.xml:
cd config
gedit config.xml
Update the value between the id tags to the copied ID.
<id>my-new-id-value-is-set-here-12345678</id>
Versions aion-v0.1.9 and later do not require generating an id. A temporary unique id will be assigned to your kernel at runtime.