Hi clayti
What you need to do is examine the Single Source Layout
properties. If you are using version 7, click the Edit... button on
the dialog. If it is version 6 or older, the button is labeled
Advanced...
Either way, the next dialog you see should have a TOC Styles
tab. Click that.
This should list a lot of TOC styles you may apply. The one
you are interested in is labeled "Single Click to Open Books".
Select that and dismiss all dialogs. Re-compile and you should be
golden.
And this only will work for compiled CHM output. WebHelp is
an equine of a different hue.
Cheers... Rick