When attaching a mobile device to my MAC, XDK does not recognise its existence under the debug tab.
I have to restart XDK with the device attached for it to be recognised, thereafter unplugging and plugging back in the device and XDK continues to recognise it.
If XDK is started without a device attached, then I have to restart XDK with the device attached for debug to work.
There could be a conflict with differing versions of adb on your system, which we try to mitigate with some fancy footwork inside the XDK, but it still happens. Also, changing the USB connection type in your Android device can make a difference. There are two types of connections: MTP and PTP (media and camera). They go by different names on different devices and the way you switch between them varies with vendor and Android version, but switching to the other connection type may help. I generally have best luck if the device is configure to connect in PTP (or camera) mode, but it will vary from device to device.
I have seen a few devices where it is either impossible or very difficult to switch between the USB connection modes, especially in 4.x devices. Sometimes you have to find the setting in the settings section, usually there is a USB notification icon that you can pulldown and touch to make the change. Confirm that you can get the Debug connection working with another device, so you can figure out if it is the device or something on your Mac. See this page for more help > https://software.intel.com/en-us/xdk/docs/configuring-your-windows-usb-android-debug-connection-for-... < (despite the URL name, it also applies to Macs).
Under USB connection methods MTP is described as Lets you transfer media files on Windows or using Android File Transfer on Mac. There is another option labeled USB flash drive, described as Use you computer to transfer SD card files, you phone cannot access the SD card in this mode.
Tha last option is Charge only.