Apple's "extend" setting is a proprietary function that is designed to only work with other Apple routers.
Unfortunately, it is extremely unlikely that it would work with Fios / Actiontec devices (or any other router for that matter) if you are trying to "extend" using wireless only.
If you connect the Express to one of the LAN <-> ports on the Actiontec router using a permanent, wired Ethernet connection, the Express can be configured to provide more wireless coverage that way.
That would work if the Time Capsule is now connected to the Actiontec router using a permanent wired Ethernet connection.
This can get complicated though, since having two routers in close proximity to each other with both providing a wireless signal can introduce interference effects that limit the performance of both routers.
In cases like this, it is always a good idea to turn off the wireless on one of the routers if possible.