To enable the models, you need a license for DS-5 Professional Edition (Cortex-A8, Cortex-A9x4) or Ultimate Edition (Cortex-A8, Cortex-A9x4 and ARMv8 FVP).
Provided that you installed Community Edition as a "standalone", rather than as Eclipse plugins, you can go to Help > ARM License Manager... > Add License... and then select "Generate a 30-day evaluation license". This will allow you to use DS-5 Professional Edition for 30-days. Finally, use the dropdown under "Select the toolkit that you intend to use" to change the version to Professional Edition, then restart Eclipse as prompted.
If you're interested in trying the ARMv8 support available in Ultimate Edition, you can request a trial: Try DS-5 Ultimate Edition | ARM DS-5 Development Studio
Extra models can be purchased separately from ARM.
If you have any other questions don't hesitate to ask.