Last Updated on 2015-04-07.
Physical Android devices can be used for debugging with Android Studio easily, if they are included in the Google USB driver. The HP Slate 21 (and Pro) is not, but it can be added.
Download Google USB driver.
Unzip the file and open android_winusb.inf e.g. with Windows Notepad.
Add the lines below to the sections [Google.NTx86] and [Google.NTamd64]:
;slate21 %SingleAdbInterface% = USB_Install, USB\VID_0955&PID_7100 %CompositeAdbInterface% = USB_Install, USB\VID_0955&PID_7100&MI_01
Install the driver via Device Manager (manually).
If you are using Windows 7 or 8 upwards, you might have to disable driver signature verification.