How can I play the drums with my computer keyboard?

The plugin itself can't register keyboard inputs, because all key commands are received and managed by the DAW.

However, some DAWs offer a native "virtual MIDI keyboard" which can be played with the mouse or computer keyboard while the window is open.

In Reaper you can bring it up by going to:
View -> Virtual MIDI Keyboard

In Cubase you can bring it up by going to:
Studio -> On-Screen Keyboard

If the virtual keyboard of your DAW doesn't suffice you and you are looking for something more customizable in size and mapping, download and install the following tools and afterwards follow the video.

Download LoopBe1 virtual MIDI device
(scroll down on the page)

Download Virtual MIDI Keyboard software

The video shows how to set these tools up once they are installed. In the example Reaper is used as a DAW, but it works similar in other DAWs, once you set "LoopBe Internal MIDI" as a MIDI input device for your MIDI track.