<p>I need to add a web part zone to a wiki page. I'm opening the page using SharePoint Designer, but there doesn't seem to be an obvious way (such as a menu) to add a Web Part Zone.</p>

from:
http://office.microsoft.com/en-us/sharepointdesigner/HA101513941033.aspx
Insert a Web Part zone
In Office SharePoint Designer 2007, open the page where you want to
  insert the Web Part zone.
If the Web Parts task pane is not already open, open it by clicking
  Web Parts on the Task Panes menu.
In Design view, click the location on the page where you want to
  insert the Web Part zone.
At the bottom of the Web Parts task pane, click New Web Part Zone.
The new Web Part zone is inserted on the page.
  ...