User registration and login bridge between WordPress and Simple Machine Forum
Obviously, you need to have a SimpleMachine Forum installation working. It is preferred to have the forum installed as a subdirectory within your WordPress install. For example, if you access your blog from http://www.mydomain.com, it is preferred for you to access your forum at http://www.mydomain.com/forum. The name of the directory does not matter, but your forum and WordPress installations should not be on different domains or subdomains! Your blog on http://www.mydomain.com and forum on forum.mydomain.com will not work!
The databases do not need to be the same between the forum and WordPress installs, nor do the database users, but it is highly recommended for performance that you do so.
From that point forth, configuration is rather simple - all you need to do is install the plugin and enter the relative path to your forum install. In the above example, a WordPress install on http://www.mydomain.com and forums on http://www.mydomain.com/forum, the relative path would simply be forum/. Do not add a leading slash, and the trailing slash is optional - the plugin will assume one should be there if you do not explicitly add it.
If this is a new install, you're all set. If this is not a new install, you will probably want to synchronize your users. Please note that synchronizing your users on a site that is not a new install is not recommended, as it has the potential to completely mess up your user accounts. For more information on this process, refer to SynchronizingUsers on the Google Code website. If you have an existing WordPress site, but a fresh SMF install, you should have no issues with synchronizing users - the main problem comes when you try to synchronize both SMF and WordPress user tables together.