Some users have had messages about missing fonts in all iWork applications - Numbers and Keynote as well as Pages. In many cases these messages seem innocent but annoying. In some cases the problem can affect the layout of the document.
In Mac OS X 10.4 (Tiger), it seems the most efficient fix is the following: Delete the folder /Library/Caches/com.apple.ATS (that is the font cache) and restart.
In Mac OS X 10.5 (Leopard), it is more delicate to delete the font cache, but you can follow the instructions here.
However, that only works if the font cache is corrupted. Sometimes the font is really missing. Pages can be very picky about fonts, even fonts you do not visibly use in a document. It is for example possible that a Style you do not use contains a font that is missing. It may also be part of a bullet list.
From Pages 3.0 there is a built in fix, which may work for you. Just follow the warnings and instructions on the screen and replace the font with another one.
However, be aware that the replacement is stored in the document, when you save it. Let's assume that you get a Pages document from a friend. It gives you a warning that the font "Nice Font" is missing. You follow the instructions and choose to replace it with Times. The next day, your friend sends you "Nice Font" so you will be able to see the document as he intended it. However, as you accepted the replacement, you will no longer be able to tell which parts of the text should have been written with "Nice Font".
Besides, reports in Apple's discussion forum show that it does not always work.
Here is a list of possible fixes in no particular order, so you can try them in any order you want.
* Delete the folder /Library/Caches/com.apple.ATS (that is the font cache) and restart.
* Use Applications > Font Book to make sure you really have all the fonts that Pages complain about. If you do not, get them.
* Enable all fonts in Font Book. Disable a few to make FontBook refresh the font cache. If it then works in Pages, enable all fonts you need.
* If you think the missing fonts may come from MS Office you can go to Mactopia to download a test drive, which will install the fonts.
* If you think the missing fonts come from Pages, reinstall Pages.
* Log into another account on your computer. That may clean up some settings and make the problem go away.
* In Font Book, highlight the fonts Pages complains about, and go to File Validate, to make sure they are ok. Make sure there are no duplicates active.
* In the Finder, highlight the files of the fonts that Pages complains about. Drag them from the Fonts folder to the desktop and then back again to the Fonts folder.
* You can also try Applications > Utilities > Disk Utility > Repair Permissions. (To be honest, I do not think I have ever heard of this actually fixing any problem, but it is theoretically possible.)
* Some people have reported solving the problem by "cleaning" the document saving it to another format. Try for example saving your document as an earlier version of Pages and then reopen it. Warning! You may lose formatting here, so keep the original as back up.
* Restart.
* Log feedback to Apple complaining about this. With enough feedback, they may fix this for future versions of Pages.
For problems with Helvetica Narrow, see this post.