Which version of AirPort Utility should I download for OS X Yosemite v.10.10.3?

None.

You already have the most up to date version installed on your Mac.

Look in Finder > Applications > Utilities > AirPort Utility

I had the same question/problem. The Airport Utility in my Utility folder says Vers 6.3.5 even after I updated to Yosemite. When trying to open it, I get a message that says not supported and go to Apple.com/support/airport, but I do not see a download for Yosemite OS X 10.10. Apple should include an update for Airport in the many Software Updates reminders they send out.

But here is my problem:

My old original Airport Express isn't recognized on my Mac running Yosemite. But oddly, the Airport Express now won't work with my iPad 1 either. I mostly use my iPad to connect to the Airport and simply use the Music program to choose the Airport network directly for playing music on the stereo. It worked brilliantly until I connected my iPad 1 to my Mac. Interestingly there were no updates for the iPad since it's so old, but do you think somehow Yosemite imbedded some software in the old iPad that is conflicting with the Airport? Time to buy a new Airport if they make new ones?

Thanks.

sorry, duplicate reply

The Airport Utility in my Utility folder says Vers 6.3.5 even after I updated to Yosemite.

AirPort Utility 6.3.5 is the most up to date version of AirPort Utility for your Mac.

Apple should include an update for Airport in the many Software Updates reminders they send out.

The most up to date version of AirPort Utility is automatically included with any new operating system or operating system update. The reason for this is that AirPort Utility is now part of the operating system. It is not a separate application. This has been the case for about the last 4 years.

My old original Airport Express isn't recognized on my Mac running Yosemite

Please check on the side of the Express for the model number of the device. It is hard to see, but starts with an "A" followed by four numbers.

If you have A1084 or A1088, newer 6.x versions of AirPort Utility are not compatible with the older versions of the AirPort Express (and AirPort Extreme). Another way of saying the same thing is that Apple discontinued support of the older AirPorts when they introduced the Lion operating system about 4 years ago.

The A1084 and A1088 models of the Express are considered Obsolete by Apple. So, if you call Apple for help on one of these models, they will not be able to help you, even if you are willing to pay.

But, you can still use the older AirPort if you have....or can borrow for a few minutes.....an older Mac running Leopard or Snow Leopard, or you have a PC with AirPort Utility installed on the device.