The JW Player WordPress Plugin enables you to configure and embed audio and video content through your WordPress website. This is the official JW Player plugin, developed by the JW Player team itself. Key features include:
Before installing the JW Player plugin, please ensure your WordPress installation meets the following requirements:
To install the plugin, extract the jw-player-plugin-for-wordpress.zip and FTP the folder to your WordPress plugins directory (/wp-content/plugins/) on your web server.
Next, login to your WordPress website and navigate to Site Admin » Plugins. The JW Player Plugin will appear in the list of plugins. Click the Activate link to enable the plugin:

At this point, the plugin is ready to be used. However, if you own a commercial JW Player license, you should set your Player Edition and License Key in the JW Player » Settings screen:

Setting these options will un-lock the edition's additional features in both player and plugin, as well as hide the JW Player logo from the player video screen.
If you've previously installed the JW Player WordPress plugin, but are interested in migrating from JW Player 5 to JW Player 6, please see our Upgrading WordPress from JW5 to JW6 guide. It contains a list of all features that were added/removed, as well as a step-by-step walkthrough of the upgrade.
With the plugin working, it's time to embed your first video. Simply start or edit a post and hit the Add Media button. You will be presented with the familiar Media Manager (screenshot here is for 3.5):

Note the JW Player plugin has added two options to the media manager: Add External Media and Embed a Playlist. Let's not jump into those now though, but instead just upload a video. If you don't have a video to upload, just grab this one...
When the upload is completed, you're redirected to the Media Library. The video you uploaded is selected, displaying its properties at the right side. These include a title, caption and description, but also a new Thumb property and an Embed JW Player button:

The Thumb property allows you to select a JPG/PNG image from the library, featuring as poster image for the video. Setting thumbs for your videos is not required, but your embeds will definitely look nicer with them.
The Embed JW Player button inserts a a short-code into your post, rendering your video in JW Player. The drop-down menu is used to select a custom player template. Let's stick with the default for now, so hit the Embed JW Player button...
The Media Manager closes, revealing the post editor again. A JW Player Short Code is now inserted - the snippet of text between square brackets:

On the public site, this short code is replaced by an actual JW Player embed, containing your video. The mediaid option tells JW Player which item to insert (number 22 from the library). The JW Player Short Code recognizes more options (width, autostart, primary..) to tweak your embeds, but your basic video embed is hereby completed!
On certain server configurations, the plugin activation process not be successful due to your wp-content/uploads folder not being writable or your WordPress installation having insufficient privileges. For more information on setting file permissions please refer to this WordPress Codex entry.
If you're having another issues, please let us know through our WordPress Plugin Support Forums. We can't fix any issues you're having if we don't know they exist.
Please include a description of the problem, a link to your site demonstrating the issue, and the host you're using. The plugin has been tested on 1and1, MediaTemple, Dreamhost, and GoDaddy.
Keep an eye on the Changelog on the WordPress site if you're looking for a specific bugfix of feature enhancement for the plugin.