Installing SimplePortal makes a small change to the Navigation Menu Buttons. If SimplePortal is enabled, and in Front Page or Standalone mode:
* The Home button now links to the portal page (otherwise, this button links to the board index)
* A Forum button now links to the board index (otherwise, this button is not displayed)
 
It is the job of the themes to display these buttons. If the themes are doing this in the standard way, the SimplePortal installation will take care of the buttons for the default theme for SMF 1.1.x, and for ALL themes for SMF 2. If you are using SMF 1.1.x, and are using any themes other than the default, or if you are using SMF 2 and are using any themes that are not based on SMF2's Curve theme, you will need to do a little work by hand. But don't worry -- all the information you need is readily available.
 
If you are using SMF 1.1.X
The SimplePortal install almost always manages to make these changes correctly for the default theme. If your forum uses any other themes that have their own index.template.php, or if you get "test failed" on Themes/default/index.template.php, you will have to make changes by hand.  This is not too difficult if you follow the instructions in the General Custom Theme Tutorial -- in the SMF 1.1.x section.
 
If you are using SMF2 and CURVE-based themes
SMF2 doesn't need you to add any code to your custom themes, so the forum button should automatically work with SMF 2 and SimplePortal -- for any CURVE-based theme.If your theme has custom button images, please see the instructions in the General Custom Theme Tutorial -- in the SMF 2 section. If other mods or customizations caused any problems with the changes to Subs.php, you can find and fix the problem using the manual installation instructions.  Example: Installation Instructions for 2.0 RC3

If you are using SMF2 and non-CURVE based themes
Then you are probably using SMF 1.1.x style themes, and should follow the instructions in the
General Custom Theme Tutorial -- in the SMF 1.1.x section.  How can you easily check how your themes handle the Navigation Menu buttons? You can compare their index.template.php files to those from SMF2's Curve and SMF1.1.x's Core.  In Curve, you'll always find
foreach ($context['menu_buttons'] as $act => $button)
whereas, in Core, you'll generally find
// Show the [home] and [help] buttons.