Hi Sara,
If I understand all you really want is "Portfolio" to be highlighted in the main menu when on a single portfolio page. If I'm not mistaken this can be "faked" simply by adding some CSS to your custom.css or Quick CSS box under Angular > Theme Options > Styling.
The CSS you would want to add is:
body.single-portfolio .main_menu ul li#menu-item-407 a {
border-bottom: 2px solid #f0b70c;
}
To explain what's going on here: I've set the menu item #407 to the active state on every page that has the body class "single-portfolio" which would be all your individual portfolio pages.
Now a couple of caveats to this:
1) #menu-item-407 is the ID of "Portfolio" in the demo's menu. The number you use will be different. To find out what yours is, you'll need to view your page's source code and find the link for the portfolio page. You'll be looking for something like this:
<li id="menu-item-###" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-###"><a href="LINK-TO-YOUR-PORTFOLIO">Portfolio</a>
Once you found the number replace 407 in the code I provided above and you should be good to go.
2) The color code (#f0b70c) for the border will change depending on your website's skin. So you'll want to change that as well.
Let us know if you need help with finding your menu item ID.
Regards,
Mya