On my computer everything is on the unit D, so I'm in the same context as you.
Let's see with the standalone version. At the bottom left of the window you have three tabs :
First click on the tab "Library" if you never did it, so that you create your library by the button "Update". Let the scanner search in all your system as it is configured by default for that. Don't worry, it is quite fast.
Once done, you should now see two trees appearing in this "Library" tab:
And you're ready.
Now choose a preset and drag&drop it on the GUI. Beware, if you simply click the preset name instead of dragging it... the GUI disappears! To make it appear again, simply click the name Proteux VX which is on blue background at the top of the list, then now make carefully your drag&drop from the preset.
You can use the internal keyboard for your first tests. To make it appear, click on the little button showing a keyboard in the toolbar at the top of the windows.
Play around with this first very short tutorial... then read the excellent PDF help file. Especially starting from the page 27. The PDF help file of Proteus VX is really awesome.
If you don't have the PDF help file, don't hesitate to tell it here.