Moises,

>When I click at one link, the new page opens inside the
>left-side frame and not in the main center page.

You should put into the

< A HREF= ...

a TARGET=centerframename keyword, which references the name of the center frame that you establish with your main page code that sets up the frames.

For a sample of the TARGET keyword, right-click the upper-right message listing here in the UT and view source. You'll find TARGET=message and TARGET=_blank.

Like Ed said, any HTML book will tell you how to do this. If you prefer to learn interactively, look at a few web sites with frames and view their source.