How do I go about redirecting visitors to the main page when they enter a page that doesn't have frames?

Elaborate a bit more on the problem. I didn't quite understand what you meant.

My site uses frames. Some of my pages show up on a search engine. When a visitor clicks on that page, there are no frames. Thus, there's no way that visitor can surf the rest of the site.

Nobody knows? =\

so when the visitor clicks on a link, it redirects them to a page, but the page isn't showing the iframes. Correct?

i am really bad at javascript, but that may be your solution.
it should check to see if the window is the same as the current page reference and change to your main site area if a true value is found.

this will probably not work, but give it a go. put it in the HEAD area of one of the pages that appears inside of a frame normally. then try to visit that page directly in your browser to test it.

<script type="text/javascript">
<!--
if (window = location.href){
top.location.href='http://www.yoursite.com/';
}
// -->
</script>

If a visitor is looking at a page without frames, I want them to be redirected to a page that does.

And no, that code doesn't work.