Looks like the latest patch is causing a few plugin issues, according to this thread.
Corel User to User Web Board • View topic - Plugins not useable
I can offer you some work arounds using GIMP, which is free.
Load up the void in GIMP, then choose menu "Colors : Color to Alpha". In the dialog box, just make sure that white is the active color and run the filter. Now you'll see the image becomes transparent in the white areas.
Now, save this image with the transparency as a .PNG file. This will preserve the transparency. When I look at even the icon for the file on my desktop in Windows 7, I can see the icon also has transparency too (not sure if WinXP or others will show this).
From there, load up the PNG file in PSP, and you should have transparency retained. Add your white background layer, and you should be ready to go.
I hope this is helpful!
EDIT: PS... I'm not sure if PSP X3 has this a feature similar to "color to alpha". If so, then you can save a few steps.