How do I know which version of the firmware for airport utility I have

I have an imac and an Airport Time Machine. I just bought an airport express in order to extend the range of the WiFi in my house. Everytime the airport utility tries to configure teh airport express, I get an unexpected error message. I have checked the support site and think I need the latest firmware update fort eh airport. I have the airport utility set up to updaate automatically weekly, and upon opening the utility, but don't know if I have the latest firmware. When I click the "about airport utility" it tells me I have version 6.3.1(631.4), but I am suspicious that it is not the current version. Does anyone have any way for me to check? and does anyone have any idea why I keep getting the unexpected errormessage? thank you.