Popular live frontend posting theme P2 lets you quickly create and publish posts from frontend but by default it only provides you a text field where you can add your post, there are no options to select and add categories from the frontend posting box. This tutorial is on a simple WordPress plugin that enable you to assign a category to your post when using P2 WordPress theme.
Installation & Usage: Simply install and activate the new WordPress plugin P2 New Post Categories. After activation the plugin adds a dropdown menu of categories to the P2 new post form.