The basic idea is that you have a layout and a profile for each. Create a layout (clone the right+main+left and give it a name). Now, create a profile (call it the same thing for sanity). In that profile, go to the bottom of the setting, and choose the layout for the desktop layout dropdown option. Now, on the main template manager tab, you can select a menu item (or items) and assign to the newly created profile, which will associate them with the layout.
If you are using Joomla 2.5, the option to point pages/menu items to profiles is on the left, regardless of which of the tabs you are on, where in Joomla 1.5, you can find this setting on the Global tab in the template manager. You have to think a little bit backwards to set these up – just like in Joomla in general: you create an article THEN you point a menu item at it…in this case, you create a layout, assign a profile to it, THEN you point the pages at it.
Make sense? Does this help?
Scott Lavelle - Technical Resource Solutions, LLC
Certified Joomla Administrator