fonts
Are you opening a site you previously created with iWeb? If so, maybe you used fonts in that site that you no longer have.. You'd need to find the text in your site and change to a font that you have..
If it's with a blank site, It would be a good idea to post which ones are missing here.
There may have been some fonts installed with the iLife disc, or something from the OS X disc that iWeb is expecting to be installed..
Brian