First, change your homepage option in the main theme options to the page you want to be your homepage. Then, make sure you have a few portfolio items already created.
Now go to the theme's Template Builder section and there should be a Frontpage template already created. If not, go ahead and create a template by filling in the blank field for the new template name and then save it. Populate that template by using the dropdown and add a Portfolio element. Select the dropdown and you can choose which portfolio item categories it should display as well as the other options.
If the Frontpage template is already there, then open it and then expand the Portfolio element in it. Choose the options you'd like (making sure to pick the category that will show items you have already created).
Now go to your themes Pages and open up the page you designated as the homepage so that you are able to edit it. On the right hand side of the visual editor use the Layout meta box to select a Dynamic Template and the Frontpage one you just edited. Save the page and take a look at the frontpage.
If you are still having issues, create a temporary admin account for me and I'll log in and set up an example for you.
You can send the information to my email at DevinVinson (at) gmail.com. Make sure to include a link to this topic so that my spam filter doesn't grab it :)