In the Axon forum you will find a detailed guide (sticky topic by me) for setting it up. The guide is actually for Axon which is designed to sit between the FXO (the adapter) and the FXS (Express Talk). I assume you want ot use the FXO to connect to a physical line from your local telco to handle inbound/outbound calls (what an FXO is for)? The linksys FXO adapters usually also have 1 or 2 FXS ports which you can set-up as extensions.
The instructions aren't that simple though. The Linksys by Cisco products like the 3102 are business class networking devices that are quite sophisticated and there are literally hundreds of configurable options which makes it highly adaptable in different environements, but also a difficult to set-up for the average user.