Try the following:
1 - delete the iWeb preference file, com.apple.iWeb.plist, that resides in your
User/Home/Library/ Preferences folder.
2 - delete iWeb's cache file, Cache.db, that is located in your
User/Home/Library/Caches/com.apple.iWeb folder (Snow Leopard and Earlier).
3 - launch iWeb and try again.
NOTE: In Lion and Mountain Lion the Library folder is now invisible. To make it permanently visible enter the following in the Terminal application window: chflags nohidden ~/Library and hit the Enter button - 10.7: Un-hide the User Library folder.
OT