In Homesite, I assume you're using the Frames Wizard to create your frameset. When you get to the second page of this wizard (after you've been asked how many rows/columns you'd like) you are presented with the Frame Attributes page.
There is a text bok in this section labelled "Name". Give each of your frames meaningful names, lets say "Toolbar" and "Main".
Now, if you want to create a link on the page that loads into the ToolBar frame that will open it's link in the Main frame you'd have a link like this:
<a href="somepage.html" target="Main">Some link</a>
The key section here is the target="main" bit. This tels the browser to open this link in the frame called "Main".
If you want a link to overwrite all frames (go back to no frames at all) you'd use "_top " as the target.
Hope this helps!