If you found that you cannot select the device for debugging, maybe your target framework is higher than your existing debugging device. You can confirm this when you get this error:
"Running this project required an Android device with API 21 or above".
Because I only install the recommended device and I am still new, I finally manage to found out that all we need to do is go to the project option ( by clicking the "Configure Target API..." or go to "Project" + " 'projectName' Options" (it is under "solution Options") ) Change the Android Application's Minimum Android version and Target Android version to the lowest option, which is might be "Override - Android 1.6 (API level 4).
Go to workspace, select the device, and cheers !Happy coding.