How can I select specific a category for my por...

You can use the custom field with the name “wpex_portfolio_page_category” and then enter the ID of the portfolio category for the value field.

If you want the filter of specific categoryes, you can create a parent category and use the ID of that one so the child categories will show up as the filterable buttons.

There was a bug in WP 3.4 if your custom fields aren’t working, download the custom fields fix plugin.