In order to record using a MIDI keyboard you may want to plug the keyboard to your computer, launch MixPad and confirm your computer recognizes the MIDI keyboard. Then, click on Options on the track you want to record, click on record using MIDI device, select the keyboard you want to use. On MIDI playback options you can select different VST instruments if you wish to hear different sounds from the keyboard on the recordings.
After you have set the MIDI keyboard as your recording device, you need to click on the record button on the track, and then on the record button on the playback buttons on the bottom on of the program's window. As stated before, the recording will start once the program detects MIDI signal coming in.