Thanks for reaching out to us about this, and I'm sorry for any trouble caused by this issue. I'd like to ask a few questions to get a better idea of what is occurring. To begin, what version of macOS is your system running? In addition, would I be correct that you downloaded VoodooPad from our website?
Could you let me know if the VoodooPad icon in your dock has a dot under it? If it does, that should mean that it is active, and you can open a document by long-clicking on the icon to open a menu or by using the VoodooPad menu items in the top left of the screen. In this case, it would still be unusual that VoodooPad did not open a welcome window.
If the VoodooPad icon does not have a dot under it after clicking on it and it isn't visible at all on the Activity Monitor, it could mean one of several things:
There may be a window stating that "VoodooPad is an app downloaded from the Internet. Are you sure that you want to open it?" This window could be obscured by another window, and the "Open" button must be chosen to open VoodooPad.
The application is failing due to a corrupted download. Could you try deleting the application and re-downloading it?
That the application is failing for another reason. If none of the above approaches apply to your situation or fix the issue, please let me know and we can take a closer look.