You might want to review the System Requirements for the "ac" product, as it sounds like you are trying to set up the "ac" product using an older 5.x.x version of AirPort Utility. Don't think that will work well at all.
From the Product Specifications: http://www.apple.com/airport-extreme/specs/
Setup and Administration
- iPhone, iPad, or iPod touch with iOS 5 or later and AirPort Utility 1.3
- Mac with OS X Lion v10.7.5 or later and AirPort Utility 6.3
The Apple 802.11ac products will "bridge" together just fine, whether you are using Ethernet or wireless to connect the devices togerther.
Setup is virtually automatic using AirPort Utility 6.3 on a Mac, or the utility on an iOS device.
An Ethernet "bridge" would probably work with another brand, but I doubt that wireless would work, as Apple uses proprietary settings to "extend a wireless network". Chances are very slim that you will be able to get Apple to work with another brand of router using wireless.