Q: I have used windows to sync my i5 and ipad, just purchased a mac air but ipad is not recognized? Can someone help??

If you mean the device is not recognized in iTunes, the first thing to try is quit iTunes, restart the Mac and restart the iPad and then try again.

If you don't see the iPad in iTunes enable the sidebar by going to View>Show Sidebar from the iTunes menu. Once the sidebar is enabled, you should see the iPad under Devices in the left sidebar.

If that's not the problem and iTunes is not recognized at all, look here for help.

iOS: Device not recognized in iTunes for Mac OS X - Support - Apple

If iTunes is telling you that you that your iPad will be erased when you sync, you have to either transfer the iTunes library from the Windows machine to the Mac, or if the old computer is gone or not functioning, then you have to make the iTunes library your syncing library, by following the steps in this user tip.

https://discussions.apple.com/docs/DOC-3141

Sync Your iOS Device with a New Computer Without Losing Data

http://www.howtogeek.com/104298/sync-your-ios-device-with-a-new-computer-without -losing-data/

Syncing to a "New" Computer or replacing a "crashed" Hard Drive

https://discussions.apple.com/docs/DOC-3141

iPad not appearing in iTunes

http://www.apple.com/support/ipad/assistant/itunes/

iOS: Device not recognized in iTunes for Mac OS X

http://support.apple.com/kb/TS1591

iOS: Device not recognized in iTunes for Windows

http://support.apple.com/kb/TS1538

iTunes for Windows: Device Sync Tests

http://support.apple.com/kb/HT4235

IOS: Syncing with iTunes

http://support.apple.com/kb/HT1386

Apple - Support - iPad - Syncing

http://www.apple.com/support/ipad/syncing/

 Cheers, Tom