On MOSS, I created a new site collection by using the blank site
template. While trying to add SummaryLinkWebPart I noticed it is not
available. There are only about 8 webparts available.

So I go to site actions -> site settings -> Web Part Gallery, then I
click on the new toolbar button to select the webparts I want to
populate the gallery. After the webpart is in the gallery, I get the
following message saying "Unable to display this Web Part. To
troubleshoot the problem, open this Web page in a Windows SharePoint
Services-compatible HTML editor such as Microsoft Office SharePoint
Designer. If the problem persists, contact your Web server
administrator."

What do I miss here?