Hi Crazybiker,
You can only apply effects to MIDI clips if you are using a virtual instrument (VSTi) to synthesize the MIDI into audio. Virtual instruments are modules that will take MIDI input and turn it into a standard audio signal that we can hear. Any MixPad tracks which are assigned to use a VSTi will also have any effects applied.
If you are not using VSTi plugins to synthesize your MIDI, then you are likely to be using Windows' built in synthesiser (the MS Wavetable Synth). This synthesizer exists outside of MixPad's audio chain and consequently MixPad can't apply effects to this audio.
There are plenty of places where you can download free VST instruments (just Google) if you would like to give this method a try.
Regards,
Ben