I believe you need this infopopup widget for it to work. See attachment.
Yas(h)ar
At the bottom of the SelectionWidget.xml there is an <info>widgets/InfoTemplates/InfoPopupWidget.swf</info> tag.
Make sure it is set to the correct path to your .swf file (remember, it is CaSe sensitive).
If that doesn't work, what does the rest of the error message say?
R_
widgets/InfoTemplates/InfoPopupWidget.swf
widgets/InfoPopup/InfoPopupWidget.swf