how do you change to bridge mode?

If you are using the Lion or Mountain Lion operating system on your Mac.....

Open Macintosh HD > Applications > Utilities > AirPort Utility

Click on the AirPort Exrreme icon, then click Edit

Click the Network tab at the top of the window

Change the setting for Router Mode from DHCP and NAT to "Off (Bridge Mode)"

Click the Update button at the lower right of the window and the AirPort will restart in 30 seconds

Then, power off your complete network, wait a minute, start the modem first and let it run a minute, then start the AirPort next the same way, then start other devices one at a time the same way.

If you are using the Leopard or Snow Leopard operating system on your Mac.......

Open Macintosh HD > Applications > Utilities > AirPort Utilty

Click on the AirPort Extreme, then click Manual Setup

Click the Internet icon

Click the Internet Connection tab

Change the setting for Connection Sharing to "Off (Bridge Mode)"

Click Update at the lower right of the window and the AirPort will restart in 30 seconds.

Then, power cycle the entire network as instructed above

When using bridge mode on the AE do I also turn my ISP to bridge mode? Or leave it DHCP?

If you use Bridge Mode on the AirPort, then the device supplied by your ISP should be using DHCP (and NAT).

If your AirPort is set to use DHCP and NAT, then the device provided by your ISP should be configured to operate in Bridge Mode.