To add your Twitter account, visit “Edit My Profile” and click “Add Twitter account” next to the Twitter field. Twitter will ask you to verify your account name and password. Once the account is verified, you’ll be asked how you’d like to share your tweets on LinkedIn (see below). Note that once your accounts are joined, you can change this setting at any time by clicking “Edit” next to your Twitter account name.
Note that in order to send tweets from Twitter to LinkedIn, your Twitter account must be set as public. Make sure the “Protect my tweets” box is not checked in your Twitter Settings.
Click here to add your Twitter account to LinkedIn.
You can choose which LinkedIn status updates you share with Twitter, as well as which tweets are displayed as your LinkedIn status. This gives you complete control over which networks see your updates and what they see, automatically or manually.
To share updates from Twitter to LinkedIn, visit the settings page. Here you have the option to share all tweets, to share only tweets that contain #in or #li, or not to share tweets at all. You can change these settings at any time be clicking “Edit” next to your Twitter account name.
To share updates from LinkedIn to Twitter, check the box next to the Twitter icon on the LinkedIn home page. The first time you do this, Twitter will verify your account name and password. Whenever the Twitter box is checked, that update will publish to your Twitter feed. Click here to edit your Twitter settings on LinkedIn.
Hashtags (#) are a way to provide additional context for your tweets. Use #in or #li to publish your Twitter updates to your LinkedIn status. Just make “#in” part of your tweet (for example, “Working on a new blog post about design optimization. #in”), and it will be displayed as your LinkedIn status. These can be added to your tweets at any time and will override your sharing settings. If your tweet is personal and you don’t wish to share it with your professional crowd, just exclude #in or #li.
Excerpt from: http://learn.linkedin.com/twitter/