Pretty self-explanatory, but this is for people who don't know how... 
Step 1:
In your eclipse file editing directory, open your project
Step 2:
Create a new folder named "libs" and extract the .jar library you want to use into it
Step 3:
Go back and open up the .classpath file
Step 4:
Add this into the .classpath file:
Code:
<classpathentry kind="lib" path="libs/mysql.jar"/>
You'll need to edit the "mysql.jar" to your jar!
Step 5:
If you have eclipse open, right click the project folder you're working on in eclipse and click "refresh". If eclipse is closed, just open eclipse
Step 6:
Congratulations, you have added a library!