Once you have downloaded the Vielklang plug-in, you will need to open it in your DAW to activate it.
The plug-in is a software instrument, so you may need to create an instrument/MIDI track as oppose to an audio track to be able to select the plug-in.
Logic Pro X:
1) Create a 'Software Instrument' track
2) Select Vielklang2 under AU Instruments > zplane
1) Create an 'Instrument' track
2) Under VST Instrument, select zplane > vielklang2
If you are still having issues, please contact technical support by clicking here.