Ok, thanks for clarifying.
Let's double-check your extended network configuration.
o Temporarily connect both the MacBook and the 802.11n AirPort Express Base Station (AXn) directly to an available LAN (opposing-arrows) port of the 802.11n AirPort Extreme Base Station (AEBSn) for the setup phase. Once done, move them to their desired locations.
o Open AirPort Utility and select the AEBSn.
o Choose Manual Setup from the Base Station menu.
o Click AirPort in the toolbar, and then click Wireless.
o Choose “Create a wireless network” from the Wireless Mode pop-up menu, and then select the “Allow this network to be extended” checkbox.
o Next, select the AXn, and choose Manual Setup.
o Choose “Extend a wireless network” from the Wireless Mode pop-up menu, and then choose the network (the one created by the AEBSn) you want to extend from the Network Name pop-up menu.
o Enter the base station network and base station password is necessary.
o Click Update to update the base station with new network settings.
(ref: Page 46 of "
Designing AirPort Networks Using AirPort Utility.)