Hello Guido,
Hm... you shouldn't have to do anything special - as soon as you open a dialog, it should be there (in top, left corner for a new installation, I think). I always try to verify that the coordinates are within the current screen. Never heard about this problem before...
You can also check the the following Registry key:
HKEY_CURRENT_USER\Software\Pelle Orinius\PellesC\Dialog Editor
value: ToolbarWindow "left, top, right, bottom, maximized?"
Pelle